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

@@ -225,9 +225,11 @@ 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);
- arg->rule = rule;
- goto out;
+ if (likely(atomic_inc_not_zero(&rule->refcnt))) {
+ arg->rule = rule;
+ goto out;
+ }
+ break;
  }
  }