perf: Fix static strings treated like dynamic ones

The raw_field_ptr() helper, used to retrieve the address of a field
inside a trace event, treats every strings as if they were dynamic
ie: having a secondary level of indirection to retrieve their
contents.

FIELD_IS_STRING doesn't mean FIELD_IS_DYNAMIC, we only need to
compute the secondary dereference for the latter case.

This fixes perf sched segfaults, bad cmdline report and may be
some other bugs.

Reported-by: Jason Baron <jbaron@redhat.com>
Reported-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Tom Zanussi <tzanussi@gmail.com>

tools/perf/util/trace-event-parse.c

@@ -1925,7 +1925,7 @@ void *raw_field_ptr(struct event *event, const char *name, void *data)
 	if (!field)
 		return NULL;
 
-	if (field->flags & FIELD_IS_STRING) {
+	if (field->flags & FIELD_IS_DYNAMIC) {
 		int offset;
 
 		offset = *(int *)(data + field->offset);