xfrm: Accept ESP packets regardless of UDP encapsulation mode

From: Martin Willi <martin@strongswan.org>

An IPsec node speaking IKEv2 MUST accept incoming UDP encapsulated
ESP packets, even if no NAT situation is detected. This is important
if MOBIKE is in use. Some implementation keep the encapsulation
mode if they move out of a NAT situation.

Signed-off-by: David S. Miller <davem@davemloft.net>