BH must be disabled when invoking nf_conncount_gc_list() to perform
garbage collection, otherwise deadlock might happen.

  nf_conncount_add+0x1f/0x50 [nf_conncount]
  nft_connlimit_eval+0x4c/0xe0 [nft_connlimit]
  nft_dynset_eval+0xb5/0x100 [nf_tables]
  nft_do_chain+0xea/0x420 [nf_tables]
  ? sch_direct_xmit+0x111/0x360
  ? noqueue_init+0x10/0x10
  ? __qdisc_run+0x84/0x510
  ? tcp_packet+0x655/0x1610 [nf_conntrack]
  ? ip_finish_output2+0x1a7/0x430
  ? tcp_error+0x130/0x150 [nf_conntrack]
  ? nf_conntrack_in+0x1fc/0x4c0 [nf_conntrack]
  nft_do_chain_ipv4+0x66/0x80 [nf_tables]
  nf_hook_slow+0x44/0xc0
  ip_rcv+0xb5/0xd0
  ? ip_rcv_finish_core.isra.19+0x360/0x360
  __netif_receive_skb_one_core+0x52/0x70
  netif_receive_skb_internal+0x34/0xe0
  napi_gro_receive+0xba/0xe0
  e1000_clean_rx_irq+0x1e9/0x420 [e1000e]
  e1000e_poll+0xbe/0x290 [e1000e]
  net_rx_action+0x149/0x3b0
  __do_softirq+0xde/0x2d8
  irq_exit+0xba/0xc0
  do_IRQ+0x85/0xd0
  common_interrupt+0xf/0xf
  </IRQ>
  RIP: 0010:nf_conncount_gc_list+0x3b/0x130 [nf_conncount]

Fixes: 2f971a8f ("netfilter: nf_conncount: move all list iterations under spinlock")
Reported-by: Laura Garcia Liebana <nevola@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>