l2tp: avoid use-after-free caused by l2tp_ip_backlog_recv

commit 51fb60eb162ab84c5edf2ae9c63cf0b878e5547e upstream.

l2tp_ip_backlog_recv may not return -1 if the packet gets dropped.
The return value is passed up to ip_local_deliver_finish, which treats
negative values as an IP protocol number for resubmission.

Signed-off-by: Paul Hüber <phueber@kernsp.in>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Willy Tarreau <w@1wt.eu>