LU-7893 osd-zfs: calls dmu_objset_disown() with NULL

LU-7893 osd-zfs: calls dmu_objset_disown() with NULL

When a ZFS-backed target is mounted, and dmu_objset_own() fails,
osd_mount() follows a goto err: and calls dmu_objset_disown(NULL).
This triggers a kernel panic due to NULL pointer dereference.

Call dmu_objset_disown() only if objset_t o->od_os is non-NULL.

Signed-off-by: Olaf Faaland <faaland1@llnl.gov>
Change-Id: I04ce39fbf97a87978c5819731ee1eba9debe6db2
Reviewed-on: http://review.whamcloud.com/19052
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>