xfrm: Introduce xfrm_input_afinfo to access the the callbacks properly

IPv6 can be build as a module, so we need mechanism to access
the address family dependent callback functions properly.
Therefore we introduce xfrm_input_afinfo, similar to that
what we have for the address family dependent part of
policies and states.

Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

net/ipv4/xfrm4_policy.c

@@ -325,6 +325,7 @@ void __init xfrm4_init(void)
 
 	xfrm4_state_init();
 	xfrm4_policy_init();
+	xfrm4_protocol_init();
 #ifdef CONFIG_SYSCTL
 	register_pernet_subsys(&xfrm4_net_ops);
 #endif