fs,eventpoll: don't test for bitfield with stack value

In case if epoll_ctl is called with operation EPOLL_CTL_DEL then
@epds.events variable allocated on stack may contain random bits which
we test then for EPOLLEXCLUSIVE.  Since currently the test look like

	if (epds.events & EPOLLEXCLUSIVE) {
		if (op == EPOLL_CTL_MOD)
			goto error_tgt_fput;
		if (op == EPOLL_CTL_ADD && (is_file_epoll(tf.file) ||
				(epds.events & ~EPOLLEXCLUSIVE_OK_BITS)))
			goto error_tgt_fput;
	}

Nothing serious will happen even if epds.events has this bit set, still
better to be on safe side and make sure that we're to test this bit at
all.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Cyrill Gorcunov <[email protected]>
Cc: Al Viro <[email protected]>
Cc: Andrey Vagin <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

fs/eventpoll.c

@@ -1895,7 +1895,7 @@ SYSCALL_DEFINE4(epoll_ctl, int, epfd, int, op, int, fd,
 	 * so EPOLLEXCLUSIVE is not allowed for a EPOLL_CTL_MOD operation.
 	 * Also, we do not currently supported nested exclusive wakeups.
 	 */
-	if (epds.events & EPOLLEXCLUSIVE) {
+	if (ep_op_has_event(op) && (epds.events & EPOLLEXCLUSIVE)) {
 		if (op == EPOLL_CTL_MOD)
 			goto error_tgt_fput;
 		if (op == EPOLL_CTL_ADD && (is_file_epoll(tf.file) ||