xfs: collapse the ->finish_item helpers

Each log item's ->finish_item function sets up a small amount of state
and calls another function to do the work.  Collapse that other function
into ->finish_item to reduce the call stack height.

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