xfs: don't bother calling xfs_refcount_finish_one_cleanup in xfs_refcount_finish_one

In xfs_refcount_finish_one we know the cursor is non-zero when calling
xfs_refcount_finish_one_cleanup and we pass a 0 error variable.  This
means xfs_refcount_finish_one_cleanup is just doing a
xfs_btree_del_cursor.

Open code that and move xfs_refcount_finish_one_cleanup to
fs/xfs/xfs_refcount_item.c.

Inspired-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>