net: fix possible deadlock in sum_frag_mem_limit

Dave Jones reported a lockdep splat occurring in IP defrag code.

commit 6d7b857d541ecd1d (net: use lib/percpu_counter API for
fragmentation mem accounting) added a possible deadlock.

Because percpu_counter_sum_positive() needs to acquire
a lock that can be used from softirq, we need to disable BH
in sum_frag_mem_limit()

Reported-by: Dave Jones <davej@redhat.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/inet_frag.h

@@ -114,7 +114,13 @@ static inline void init_frag_mem_limit(struct netns_frags *nf)
 
 static inline int sum_frag_mem_limit(struct netns_frags *nf)
 {
-	return percpu_counter_sum_positive(&nf->mem);
+	int res;
+
+	local_bh_disable();
+	res = percpu_counter_sum_positive(&nf->mem);
+	local_bh_enable();
+
+	return res;
 }
 
 static inline void inet_frag_lru_move(struct inet_frag_queue *q)