perf pmu: Fix pmu events parsing rule

Currently all the event parsing fails end up in the event_pmu rule, and
display misleading help like:

$ perf stat -e inst kill
event syntax error: 'inst'
\___ Cannot find PMU `inst'. Missing kernel support?
...

The reason is that the event_pmu is too strong and match also single
string. Changing it to force the '/' separators to be part of the rule,
and getting the proper error now:

$ perf stat -e inst kill
event syntax error: 'inst'
\___ parser error
Run 'perf list' for a list of valid events
...

Signed-off-by: Jiri Olsa <[e-mail]>
Reported-by: Ingo Molnar <[e-mail]>
Tested-by: Arnaldo Carvalho de Melo <[e-mail]>
Cc: Alexander Shishkin <[e-mail]>
Cc: David Ahern <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/util/parse-events.y

···
 	   event_bpf_file
 
 event_pmu:
-PE_NAME opt_event_config
+PE_NAME '/' event_config '/'
 {
 	struct list_head *list, *orig_terms, *terms;
 
-	if (parse_events_copy_term_list($2, &orig_terms))
+	if (parse_events_copy_term_list($3, &orig_terms))
 		YYABORT;
 
 	ALLOC_LIST(list);
-	if (parse_events_add_pmu(_parse_state, list, $1, $2, false)) {
+	if (parse_events_add_pmu(_parse_state, list, $1, $3, false)) {
 		struct perf_pmu *pmu = NULL;
 		int ok = 0;
 		char *pattern;
···
 		if (!ok)
 			YYABORT;
 	}
-	parse_events_terms__delete($2);
+	parse_events_terms__delete($3);
 	parse_events_terms__delete(orig_terms);
 	$$ = list;
 }