xfs: factor the xfs_iunlink functions

Prep work that separates the locking that protects the unlinked list
from the actual operations being performed. This also helps document
the fact they are performing list insert  and remove operations. No
functional code change.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>