xfrm: interface: enable TSO on xfrm interfaces

Underlying xfrm output supports gso packets.
Declare support in hw_features and adapt the xmit MTU check to pass GSO
packets.

Signed-off-by: Eyal Birger <eyal.birger@gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>