xfs: clean up log item accesses in xfs_qm_dqflush{,_done}

Clean up these functions a little bit before we move on to the real
modifications, and make the variable naming consistent for dquot log
items.

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