tracing: move tgid out of generic entry and into userstack

The userstack trace required the recording of the tgid entry.
Unfortunately, it was added to the generic entry where it wasted
4 bytes of every entry and was only used by one entry.

This patch moves it out of the generic field and moves it into the
only user (userstack_entry).

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

kernel/trace/trace_output.c

@@ -407,7 +407,7 @@ seq_print_userip_objs(const struct userstack_entry *entry, struct trace_seq *s,
 		 * since individual threads might have already quit!
 		 */
 		rcu_read_lock();
-		task = find_task_by_vpid(entry->ent.tgid);
+		task = find_task_by_vpid(entry->tgid);
 		if (task)
 			mm = get_task_mm(task);
 		rcu_read_unlock();