LUDOC-161 zfs: Backup/Restore of ZFS backend filesystems

LUDOC-161 zfs: Backup/Restore of ZFS backend filesystems

This patch documents how to backup/restore ZFS backend
filesystems at the file system level using 'tar'.  The
existing section on backing up an OST or MDT at the
filesystem level was updated to capture zfs.  Also, a
section has been added to discuss migrating between ZFS
and ldiskfs backends using the filesystem level
backup/restore process.

This work has landed to master under LU-7585.

Signed-off-by: Joseph Gmitter <joseph.gmitter@intel.com>
Change-Id: I20619e42c7313657599788a23caf4e867f714db6
Reviewed-on: https://review.whamcloud.com/30172
Reviewed-by: James Nunez <james.a.nunez@intel.com>
Tested-by: Jenkins