xfs: refactor xlog_state_clean_iclog

Factor out a few self-contained helpers from xlog_state_clean_iclog, and
update the documentation so it primarily documents why things happens
instead of how.

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>