xfrm: add UDP encapsulation port in migrate message

Add XFRMA_ENCAP, UDP encapsulation port, to km_migrate announcement
to userland. Only add if XFRMA_ENCAP was in user migrate request.

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>