Cleanup zvol initialization code

The following error will occur on some (possibly all) kernels because
blk_init_queue() will try to take the spinlock before we initialize it.

[    5.538871] BUG: spinlock bad magic on CPU#0, zpool/4054
[    5.538885]  lock: 0xffff88021a73de60, .magic: 00000000, .owner:
<none>/-1, .owner_cpu: 0
[    5.538888] Pid: 4054, comm: zpool Not tainted 3.9.3 #11
[    5.538890] Call Trace:
[    5.538898]  [<ffffffff81478ef8>] spin_dump+0x8c/0x91
[    5.538902]  [<ffffffff81478f1e>] spin_bug+0x21/0x26
[    5.538906]  [<ffffffff812da097>] do_raw_spin_lock+0x127/0x130
[    5.538911]  [<ffffffff81253301>] ? zvol_probe+0x91/0xf0
[    5.538914]  [<ffffffff8147d851>] _raw_spin_lock_irq+0x21/0x30
[    5.538919]  [<ffffffff812c2c1e>] cfq_init_queue+0x1fe/0x350
[    5.538922]  [<ffffffff81253360>] ? zvol_probe+0xf0/0xf0
[    5.538926]  [<ffffffff812aacb8>] elevator_init+0x78/0x140
[    5.538930]  [<ffffffff812b2677>] blk_init_allocated_queue+0x87/0xb0
[    5.538933]  [<ffffffff81253360>] ? zvol_probe+0xf0/0xf0
[    5.538937]  [<ffffffff812b26d5>] blk_init_queue_node+0x35/0x70
[    5.538941]  [<ffffffff812b271e>] blk_init_queue+0xe/0x10
[    5.538944]  [<ffffffff8125211b>] __zvol_create_minor+0x24b/0x620
[    5.538947]  [<ffffffff81253264>] zvol_create_minors_cb+0x24/0x30
[    5.538952]  [<ffffffff811bd9ca>] dmu_objset_find_spa+0xea/0x510
[    5.538955]  [<ffffffff81253240>] ? zvol_free+0x60/0x60
[    5.538958]  [<ffffffff811bda71>] dmu_objset_find_spa+0x191/0x510
[    5.538962]  [<ffffffff81253240>] ? zvol_free+0x60/0x60
[    5.538965]  [<ffffffff81253ea2>] zvol_create_minors+0x92/0x180
[    5.538969]  [<ffffffff811f8d80>] spa_open_common+0x250/0x380
[    5.538973]  [<ffffffff811f8ece>] spa_open+0xe/0x10
[    5.538977]  [<ffffffff8122817e>] pool_status_check.part.22+0x1e/0x80
[    5.538980]  [<ffffffff81228a55>] zfsdev_ioctl+0x155/0x190
[    5.538984]  [<ffffffff8116a695>] do_vfs_ioctl+0x325/0x5a0
[    5.538989]  [<ffffffff81163f1d>] ? final_putname+0x1d/0x40
[    5.538992]  [<ffffffff8116a950>] sys_ioctl+0x40/0x80
[    5.538996]  [<ffffffff814812c9>] ? do_page_fault+0x9/0x10
[    5.539000]  [<ffffffff81483929>] system_call_fastpath+0x16/0x1b
[    5.541118]  zd0: unknown partition table

We fix this by calling spin_lock_init before blk_init_queue.

The manner in which zvol_init() initializes structures is
suspectible to a race between initialization and a probe on a zvol. We
reorganize zvol_init() to prevent that.

Lastly, calling zvol_create_minors(NULL) in zvol_init() does nothing
because no pools are imported, so we remove it.

Signed-off-by: Richard Yao <ryao@gentoo.org>
Fixing a bug related to set_capacity() before add_disk().

See these references:
  http://osdir.com/ml/linux.enbd.general/2002-10/msg00176.html
  http://osdir.com/ml/linux.enbd.general/2002-10/msg00177.html
  http://stackoverflow.com/questions/13518404/add-disk-hangs-on-insmod
Illumos #3006

3006 VERIFY[S,U,P] and ASSERT[S,U,P] frequently check if first argument
     is zero

Reviewed by Matt Ahrens <matthew.ahrens@delphix.com>
Reviewed by George Wilson <george.wilson@delphix.com>
Approved by Eric Schrock <eric.schrock@delphix.com>

References:
  illumos/illumos-gate@fb09f5aad449c97fe309678f3f604982b563a96f
  https://illumos.org/issues/3006

Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>
Import zfs_holey() from Open Solaris

This is prerequisite to adding support for SEEK_DATA/SEEK_HOLE to the
ZPL. It is split into its own commit for proper attribution.

Signed-off-by: Richard Yao <ryao@gentoo.org>
Reviewed-by: Richard Laager <rlaager@wiktel.com>
SEEK_DATA/SEEK_HOLE support

This adds SEEK_DATA/SEEK_HOLE to lseek()/llseek().

This is a heavily revised version of the patch proposed in
zfsonlinux/zfs#1516. The original message by Li Dongyang was:

```
Add SEEK_DATA/SEEK_HOLE by creating our own zpl_file_llseek().
Tested with xfstests 285 and 286. test0[7-9] of 285 will try to reserve
the blocks first via fallocate(2), we will fail there as currently we
don't support that.
```

My revisions address some issues that I spotted and also some things
that were spotted upon review by Richard Laager. The behavior on errors
has been altered to match the Solaris version, which is where this
extension was invented. It should still pass the XFS tests, but I am not
setup to verify that. The zfs_holey() function from Open Solaris was
moved to its own commit for attribution purposes.

Note that the filp->f_lock spinlock did not exist prior to Linux 2.6.30,
but we avoid the need for autotools check by virtue of the fact that
SEEK_DATA/SEEK_HOLE support was not added until Linux 3.1.

Closes zfsonlinux/zfs#1384

Original-patch-by: Li Dongyang <dongyang.li@anu.edu.au>
Signed-off-by: Richard Yao <ryao@gentoo.org>
Reviewed-by: Richard Laager <rlaager@wiktel.com>
Use kmem_cache for zil_vdev_node_t objects

Signed-off-by: Richard Yao <ryao@gentoo.org>
Use bitfields for boolean values in znode_t

znode_t uses 1312 bytes in length on Linux 3.9.6 amd64. Switching to
bitfields for boolean values reduces the size to 1296 bytes. Further
reductions should be possible if padding were removed, but saving 16
bytes per struct can translate into megabytes of savings when hundreds
of thousands of znodes are cached. While the savings is relatively
small in comparison to memory used, the small efficiency gain is
practically free.

Signed-off-by: Richard Yao <ryao@gentoo.org>
Remove unused option -r from zpool

Signed-off-by: Richard Yao <ryao@gentoo.org>
Implement -t option to zpool import for temporary pool names

Specifying newname to a pool import currently changes the pool name. We
implement option -t. Specifying option -t in conjuction with newpool to
zpool import will make the name change temporary. In specific, the
original pool name will be used in all label updates and therefore is
retained upon export.

This is useful in any situation where maintenance on a system's root
pool is done from another system. A good example is environments using
virtualization, where it is more desireable to do rollback of the root
dataset of a guest's root pool rather than rollback of the zvol hosting
the guest's root pool when an issue prevents the guest from booting
properly. Other examples include maintenance on root pools hosted on
iSCSI and operating system installations on USB keys that are used to
rescue non-bootable sytems.

Signed-off-by: Richard Yao <ryao@gentoo.org>