ipv4/fib: use synchronize_net() when holding RTNL

tnode_free() should use synchronize_net()
instead of syncronize_rcu() to release RTNL sooner.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/fib_trie.c

@@ -501,7 +501,7 @@ static void tnode_free(struct key_vector *tn)
 
 	if (tnode_free_size >= READ_ONCE(sysctl_fib_sync_mem)) {
 		tnode_free_size = 0;
-		synchronize_rcu();
+		synchronize_net();
 	}
 }