xfrm: Silence warnings triggerable by bad packets

After the elimination of inner modes, a couple of warnings that
were previously unreachable can now be triggered by malformed
inbound packets.

Fix this by:

1. Moving the setting of skb->protocol into the decap functions.
2. Returning -EINVAL when unexpected protocol is seen.

Reported-by: Maciej <Żenczykowski&lt;maze@google.com>
Fixes: 5f24f41e ("xfrm: Remove inner/outer modes from input path")
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Reviewed-by: Maciej Żenczykowski <maze@google.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>