ipv6: If neigh lookup fails during icmp6 dst allocation, propagate error.

Don't just succeed with a route that has a NULL neighbour attached.
This follows the behavior of addrconf_dst_alloc().

Allowing this kind of route to end up with a NULL neigh attached will
result in packet drops on output until the route is somehow
invalidated, since nothing will meanwhile try to lookup the neigh
again.

A statistic is bumped for the case where we see a neigh-less route on
output, but the resulting packet drop is otherwise silent in nature,
and frankly it's a hard error for this to happen and ipv6 should do
what ipv4 does which is say something in the kernel logs.

Signed-off-by: David S. Miller <davem@davemloft.net>