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: e659ba31d806 ("ipv6: Handle all fib6_nh in a nexthop in exception handling")
Signed-off-by: David Ahern <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/ipv6/route.c

···
 		nexthop_for_each_fib6_nh(from->nh, fib6_nh_find_match, &arg);
 
 		if (!arg.match)
-			return;
+			goto unlock;
 		fib6_nh = arg.match;
 	} else {
 		fib6_nh = from->fib6_nh;