ipv6: have a single rcu unlock point in __ip6_rt_update_pmtu

Simplify the unlock path in __ip6_rt_update_pmtu by using a
single point where rcu_read_unlock is called.

Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>