mm/page-writeback.c: place "not" inside of unlikely() statement in wb_domain_writeout_inc()

The likely/unlikely profiler noticed that the unlikely statement in
wb_domain_writeout_inc() is constantly wrong.  This is due to the "not"
(!) being outside the unlikely statement.  It is likely that
dom->period_time will be set, but unlikely that it wont be.  Move the
not into the unlikely statement.

Link: http://lkml.kernel.org/r/20170206120035.3c2e2b91@gandalf.local.home
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/page-writeback.c

@@ -580,7 +580,7 @@ static void wb_domain_writeout_inc(struct wb_domain *dom,
 	__fprop_inc_percpu_max(&dom->completions, completions,
 			       max_prop_frac);
 	/* First event after period switching was turned off? */
-	if (!unlikely(dom->period_time)) {
+	if (unlikely(!dom->period_time)) {
 		/*
 		 * We can race with other __bdi_writeout_inc calls here but
 		 * it does not cause any harm since the resulting time when