xfrm: place af number into xfrm_mode struct

This will be useful to know if we're supposed to decode ipv4 or ipv6.

While at it, make the unregister function return void, all module_exit
functions did just BUG(); there is never a point in doing error checks
if there is no way to handle such error.

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