fib: use atomic_inc_not_zero() in fib_rules_lookup

It seems we dont use appropriate refcount increment in an
rcu_read_lock() protected section.

fib_rule_get() might increment a null refcount and bad things could
happen.

While fib_nl_delrule() respects an rcu grace period before calling
fib_rule_put(), fib_rules_cleanup_ops() calls fib_rule_put() without a
grace period.

Note : after this patch, we might avoid the synchronize_rcu() call done
in fib_nl_delrule()
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/fib_rules.c

@@ -225,10 +225,12 @@ int fib_rules_lookup(struct fib_rules_ops *ops, struct flowi *fl,
 			err = ops->action(rule, fl, flags, arg);
 
 		if (err != -EAGAIN) {
-			fib_rule_get(rule);
+			if (likely(atomic_inc_not_zero(&rule->refcnt))) {
 				arg->rule = rule;
 				goto out;
 			}
+			break;
+		}
 	}
 
 	err = -ESRCH;