ipv6: Create cleanup helper for fib6_nh

Move the fib6_nh cleanup code to a new helper, fib6_nh_release.

Signed-off-by: David Ahern <dsahern@gmail.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>