perf probe: Add warning message if there is unexpected event name

This improve the error message so that user can know event-name error
before writing new events to kprobe-events interface.

E.g.
   ======
   #./perf probe -x /lib64/libc-2.25.so malloc_get_state*
   Internal error: "malloc_get_state@GLIBC_2" is an invalid event name.
     Error: Failed to add events.
   ======
Reported-by: NArnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: NMasami Hiramatsu <mhiramat@kernel.org>
Acked-by: NRavi Bangoria <ravi.bangoria@linux.vnet.ibm.com>
Reviewed-by: NThomas Richter <tmricht@linux.vnet.ibm.com>
Tested-by: NArnaldo Carvalho de Melo <acme@redhat.com>
Cc: Paul Clarke <pc@us.ibm.com>
Cc: bhargavb <bhargavaramudu@gmail.com>
Cc: linux-rt-users@vger.kernel.org
Link: http://lkml.kernel.org/r/151275040665.24652.5188568529237584489.stgit@devboxSigned-off-by: NArnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/probe-event.c

@@ -2625,6 +2625,14 @@ static int get_new_event_name(char *buf, size_t len, const char *base,
 
 out:
 	free(nbase);
+
+	/* Final validation */
+	if (ret >= 0 && !is_c_func_name(buf)) {
+		pr_warning("Internal error: \"%s\" is an invalid event name.\n",
+			   buf);
+		ret = -EINVAL;
+	}
+
 	return ret;
 }