xfrm: Support UDP encapsulation in packet offload mode

Since mlx5 supports UDP encapsulation in packet offload, change the XFRM
core to allow users to configure it.

Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Acked-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>