xfrm: prefer family stored in xfrm_mode struct

Now that we have the family available directly in the
xfrm_mode struct, we can use that and avoid one extra dereference.

Signed-off-by: Florian Westphal <fw@strlen.de>
Reviewed-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>