xfrm: choose protocol family by skb protocol

xfrm: choose protocol family by skb protocol

We need to choose the protocol family by skb->protocol. Otherwise we
call the wrong xfrm{4,6}_local_error handler in case an ipv6 sockets is
used in ipv4 mode, in which case we should call down to xfrm4_local_error
(ip6 sockets are a superset of ip4 ones).

We are called before before ip_output functions, so skb->protocol is
not reset.

Cc: Steffen Klassert <[email protected]>
Acked-by: Eric Dumazet <[email protected]>
Signed-off-by: Hannes Frederic Sowa <[email protected]>
Signed-off-by: Steffen Klassert <[email protected]>