ipv6: icmp6: add drop reason support to icmpv6_echo_reply()

Change icmpv6_echo_reply() to return a drop reason.

For the moment, return NOT_SPECIFIED or SKB_CONSUMED.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>