xfs: merge the ->log_item defer op into ->create_intent

These are aways called together, and my merging them we reduce the amount
of indirect calls, improve type safety and in general clean up the code
a bit.

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>