xfs: factor iclog state processing out of xlog_state_do_callback()

The iclog IO completion state processing is somewhat complex, and
because it's inside two nested loops it is highly indented and very
hard to read. Factor it out, flatten the logic flow and clean up the
comments so that it much easier to see what the code is doing both
in processing the individual iclogs and in the over
xlog_state_do_callback() operation.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>