ipv6: Fix unbalanced rcu locking in rt6_update_exception_stamp_rt

The nexthop path in rt6_update_exception_stamp_rt needs to call
rcu_read_unlock if it fails to find a fib6_nh match rather than
just returning.

Fixes: e659ba31 ("ipv6: Handle all fib6_nh in a nexthop in exception handling")
Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>