xfrm: extend MIGRATE with UDP encapsulation port

Add UDP encapsulation port to XFRM_MSG_MIGRATE using an optional
netlink attribute XFRMA_ENCAP.

The devices that support IKE MOBIKE extension (RFC-4555 Section 3.8)
could go to sleep for a few minutes and wake up. When it wake up the
NAT mapping could have expired, the device send a MOBIKE UPDATE_SA
message to migrate the IPsec SA. The change could be a change UDP
encapsulation port, IP address, or both.

Reported-by: Paul Wouters <pwouters@redhat.com>
Signed-off-by: Antony Antony <antony@phenome.org>
Reviewed-by: Richard Guy Briggs <rgb@tricolour.ca>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>