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>