Improve OpenRC init script

The current zfs OpenRC script's dependencies cause OpenRC to attempt to
unmount ZFS filesystems at shutdown while things were still using them,
which would fail. This is a cosmetic issue, but it should still be
addressed. It probably does not affect systems where the rootfs is a
legacy filesystem, but any system with the rootfs on ZFS needs to run
the ZFS init script after the system is ready to shutdown filesystems.

OpenRC's shutdown process occurs in the reverse order of the startup
process. Therefore running the ZFS shutdown procedure after filesystems
are ready to be unmounted requires running the startup procedure before
fstab. This patch changes the dependencies of the script to expliclty
run before fstab at boot when the rootfs is ZFS and to run after fstab
at boot whenever the rootfs is not ZFS. This should cover most use
cases.

The only cases not covered well by this are systems with legacy
root filesystems where people want to configure fstab to mount a non-ZFS
filesystem off a zvol and possibly also systems whose pools are stored
on network block devices. The former requires that the ZFS script run
before fstab, which could cause ZFS datasets to mount too early and
appear under the fstab mount points. The latter requires that the ZFS
script run after networking starts, which precludes the ability to store
any system information on ZFS. An additional OpenRC script could be
written to handle non-root pools on network block devices, but that will
depend on user demand and developer time.

Signed-off-by: Richard Yao <ryao@cs.stonybrook.edu>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1479
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

Requires:
  zfsonlinux/spl@1c6d149feb4033e4a56fb987004edc5d45288bcb

Ported-by: Tim Chase <tim@chase2k.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1509
Illumos #3552, #3564

3552 condensing one space map burns 3 seconds of CPU in spa_sync() thread
3564 spa_sync() spends 5-10% of its time in metaslab_sync() (when not condensing)
Reviewed by: Adam Leventhal <ahl@delphix.com>
Reviewed by: Dan Kimmel <dan.kimmel@delphix.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Approved by: Richard Lowe <richlowe@richlowe.net>

References:
  illumos/illumos-gate@16a4a8074274d2d7cc408589cf6359f4a378c861
  https://www.illumos.org/issues/3552
  https://www.illumos.org/issues/3564

Ported-by: Tim Chase <tim@chase2k.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1513
Fix compile warning on 32-bit systems

The definition of zfs_vdev_holder casts VDEV_HOLDER into a function pointer
passing to linux kernel's block layer function blkdev_get_by_path.
However current VDEV_HOLDER is defined to be wider than 32 bits and the compiler
warns about potential overflows. Instead of specifying different values for 32-bit and
64-bit systems using ifdefs, choose the common factor 32-bit addresses.
Redefine VDEV_HOLDER to 0x2401de7("zholder") here.

Signed-off-by: Ying Zhu <casualfisher@gmail.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1520