MFC r247187,247265,247348,247398,247540,247585,247852,248265,248267
Merge various ZFS improvements and bugfixes

MFC r247187:
  Import vendor change to avoid "unitialized variable" warnings.

  Illumos ZFS issues:
  3522 zfs module should not allow uninitialized variables

MFC r247265:
  Merge the ZFS I/O deadman thread from vendor (illumos).
  This feature panics the system on hanging ZFS I/O, helps debugging
  and resumes failed service.

  The panic behavior can be controlled with the loader-only tunables:
  vfs.zfs.deadman_enabled (enable or disable panic on stalled ZFS I/O)
  vfs.zfs.deadman_synctime (expiration time for stalled ZFS I/O)

  By default, ZFS I/O deadman is enabled by default on amd64 and i386
  excluding virtual guest machines.

MFC r247348:
  Be more verbose on ZFS deadman I/O panic
  Patch suggested upstream.

MFC r247398:
  Import metaslab_sync() speedup from vendor (illumos).

  Illumos ZFS issues:
  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)
  3578 transferring the freed map to the defer map should be constant time
  3579 ztest trips assertion in metaslab_weight()

MFC r247540:
  Fix the zfs_ioctl compat layer to support zfs_cmd size change introduced
  in r247265 (ZFS deadman thread). Both new utilities now support the old
  kernel and new kernel properly detects old utilities.

  For future backwards compatibility, the vfs.zfs.version.ioctl read-only
  sysctl has been introduced. With this sysctl zfs utilities will be able
  to detect the ioctl interface version of the currently loaded zfs module.

MFC r247585:
  Merge new read-only zfs properties from vendor (illumos)

  Illumos ZFS issues:
  3588 provide zfs properties for logical (uncompressed) space used and
       referenced

MFC r247852:
  Import ZFS bpobj bugfix from vendor.

  Illumos ZFS issues:
  3603 panic from bpobj_enqueue_subobj()
  3604 zdb should print bpobjs more verbosely

MFC r248265:
  Update zfs.8 manpage date (missing in r247585)

MFC r248267:
  Import minor ZFS changes from vendor

  Illumos ZFS issues:
  3604 zdb should print bpobjs more verbosely (fix zdb hang)
  3606 zpool status -x shouldn't warn about old on-disk format