net: ipv6: check return value of rhashtable_init

When rhashtable_init() fails, it returns -EINVAL.
However, since error return value of rhashtable_init is not checked,
it can cause use of uninitialized pointers.
So, fix unhandled errors of rhashtable_init.

Signed-off-by: MichelleJin <shjy180909@gmail.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>