tracing: Use str_has_prefix() in synth_event_create()

Since we now have a str_has_prefix() that returns the length, we can
use that instead of explicitly calculating it.

Link: http://lkml.kernel.org/r/03418373fd1e80030e7394b8e3e081c5de28a710.1549309756.git.tom.zanussi@linux.intel.com

Cc: Joe Perches <joe@perches.com>
Signed-off-by: Tom Zanussi <tom.zanussi@linux.intel.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

kernel/trace/trace_events_hist.c

@@ -1316,8 +1316,8 @@ static int synth_event_create(int argc, const char **argv)
 
 	/* This interface accepts group name prefix */
 	if (strchr(name, '/')) {
-		len = sizeof(SYNTH_SYSTEM "/") - 1;
-		if (strncmp(name, SYNTH_SYSTEM "/", len))
+		len = str_has_prefix(name, SYNTH_SYSTEM "/");
+		if (len == 0)
 			return -EINVAL;
 		name += len;
 	}