xfrm: policy: fix layer 4 flowi decoding

The commit shipped with two bugs:
 fl4->fl4_icmp_type = flkeys->icmp.type;
 fl4->fl4_icmp_type = flkeys->icmp.code;
               ~~~~ should have been "code".

But the more severe bug is that I got fooled by flowi member defines:
fl4_icmp_type, fl4_gre_key and fl4_dport share the same union/address.

Fix typo and make gre/icmp key setting depend on the l4 protocol.

Fixes: 7a020709 ("xfrm: policy: replace session decode with flow dissector")
Reported-and-tested-by: Antony Antony <antony@phenome.org>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>