net: fix possible deadlock in sum_frag_mem_limit

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

commit 6d7b857d (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>