xfrm: fix bogus WARN_ON with ipv6

net/xfrm/xfrm_input.c:378:17: warning: this statement may fall through [-Wimplicit-fallthrough=]
skb->protocol = htons(ETH_P_IPV6);

... the fallthrough then causes a bogus WARN_ON().

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Fixes: 4c203b04 ("xfrm: remove eth_proto value from xfrm_state_afinfo")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>