ipv6: Initialize the neighbour pointer of rt6_info on allocation

git commit 97cac082 (ipv6: Store route neighbour in rt6_info struct)
added a neighbour pointer to rt6_info. Currently we don't initialize
this pointer at allocation time. We assume this pointer to be valid
if it is not a null pointer, so initialize it on allocation.

Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: David S. Miller <davem@davemloft.net>