ipv6: correct return on ipv6_rcv() packet drop

The routine ipv6_rcv() uses magic number 0 for a return when it drops a
packet. This corresponds to NET_RX_SUCCESS, which is obviously
incorrect. Correct this by using NET_RX_DROP instead.

ps. It isn't exactly clear who the IPv6 maintainers are, apologies if
I've missed any.

Signed-off-by: Mark Smith <markzzzsmith@yahoo.com.au>
Signed-off-by: David S. Miller <davem@davemloft.net>