vxlan: fix the issue that neigh proxy blocks all icmpv6 packets

Commit f1fb08f6 ("vxlan: fix ND proxy when skb doesn't have transport
header offset") removed icmp6_code and icmp6_type check before calling
neigh_reduce when doing neigh proxy.

It means all icmpv6 packets would be blocked by this, not only ns packet.
In Jianlin's env, even ping6 couldn't work through it.

This patch is to bring the icmp6_code and icmp6_type check back and also
removed the same check from neigh_reduce().

Fixes: f1fb08f6 ("vxlan: fix ND proxy when skb doesn't have transport header offset")
Reported-by: Jianlin Shi <jishi@redhat.com>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Reviewed-by: Vincent Bernat <vincent@bernat.im>
Signed-off-by: David S. Miller <davem@davemloft.net>