xfs: refactor xfs_defer_finish_noroll

Split out a helper that operates on a single xfs_defer_pending structure
to untangle the code.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>