xfs: initialize the check_owner object fully

Initialize the check_owner list head so that we don't corrupt the list.
Reduce the scope of the object pointer.

Fixes: 858333dc ("xfs: check btree block ownership with bnobt/rmapbt when scrubbing btree")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>