l2tp: Fix inet_opt conversion.

We don't actually hold the socket lock at this point, so the
rcu_dereference_protected() isn't' correct.  Thanks to Eric
Dumazet for pointing this out.

Thankfully, we're only interested in fetching the faddr value
if srr is enabled, so we can simply make this an RCU sequence
and use plain rcu_dereference().

Reported-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>