ipv6: Fix may be used uninitialized warning in rt6_check

rt_cookie might be used uninitialized, fix this by
initializing it.

Fixes: c5cff856 ("ipv6: add rcu grace period before freeing fib6_node")
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: David S. Miller <davem@davemloft.net>