xfs: trace AIL manipulations

I debugging a log tail issue on a RHEL6 kernel, I added these trace
points to trace log items being added, moved and removed in the AIL
and how that affected the log tail LSN that was written to the log.
They were very helpful in that they immediately identified the cause
of the problem being seen. Hence I'd like to always have them
available for use.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Ben Myers <bpm@sgi.com>

fs/xfs/xfs_log.c

@@ -1076,6 +1076,7 @@ xlog_assign_tail_lsn_locked(
 		tail_lsn = lip->li_lsn;
 	else
 		tail_lsn = atomic64_read(&log->l_last_sync_lsn);
+	trace_xfs_log_assign_tail_lsn(log, tail_lsn);
 	atomic64_set(&log->l_tail_lsn, tail_lsn);
 	return tail_lsn;
 }