xfs: factor debug code out of xlog_state_do_callback()

Start making this function readable by lifting the debug code into
a conditional function.

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>