LU-6767 osd-zfs: Track readonly status of ZFS

LU-6767 osd-zfs: Track readonly status of ZFS

Return READONLY from osd_statfs() if underlying ZFS has been set to
READONLY, or if osd_ro() has been called.  This adds a callback for
ZFS_PROP_READONLY for when it's changed.

Signed-off-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Change-Id: Ib7f35925904b1d93f9a457936585e9783635c849
Reviewed-on: http://review.whamcloud.com/15400
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>