ipv6: start fib6 gc on RTF_CACHE dst creation

After the commit 2b760fcf ("ipv6: hook up exception table
to store dst cache"), the fib6 gc is not started after the
creation of a RTF_CACHE via a redirect or pmtu update, since
fib6_add() isn't invoked anymore for such dsts.

We need the fib6 gc to run periodically to clean the RTF_CACHE,
or the dst will stay there forever.

Fix it by explicitly calling fib6_force_start_gc() on successful
exception creation. gc_args->more accounting will ensure that
the gc timer will run for whatever time needed to properly
clean the table.

v2 -> v3:
 - clarified the commit message

Fixes: 2b760fcf ("ipv6: hook up exception table to store dst cache")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Acked-by: Wei Wang <weiwan@google.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/route.c

@@ -1340,8 +1340,10 @@ static int rt6_insert_exception(struct rt6_info *nrt,
 	spin_unlock_bh(&rt6_exception_lock);
 
 	/* Update fn->fn_sernum to invalidate all cached dst */
-	if (!err)
+	if (!err) {
 		fib6_update_sernum(ort);
+		fib6_force_start_gc(net);
+	}
 
 	return err;
 }