fs/epoll: robustify ep->mtx held checks

Insted of just commenting how important it is, lets make it more robust
and add a lockdep_assert_held() call.

Link: http://lkml.kernel.org/r/20181108051006.18751-5-dave@stgolabs.netSigned-off-by: Davidlohr Bueso <dbueso@suse.de>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Jason Baron <jbaron@akamai.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/eventpoll.c

@@ -1637,6 +1637,8 @@ static __poll_t ep_send_events_proc(struct eventpoll *ep, struct list_head *head
 	 * Items cannot vanish during the loop because ep_scan_ready_list() is
 	 * holding "mtx" during this call.
 	 */
+	lockdep_assert_held(&ep->mtx);
+
 	list_for_each_entry_safe(epi, tmp, head, rdllink) {
 		if (esed->res >= esed->maxevents)
 			break;