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

Change ndisc_recv_na() to return a drop reason.

For the moment, return PKT_TOO_SMALL, NOT_SPECIFIED
or SKB_CONSUMED. More reasons are added later.

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