netfilter: nft_limit: fix divided by zero panic

netfilter: nft_limit: fix divided by zero panic

After I input the following nftables rule, a panic happened on my system:
  # nft add rule filter OUTPUT limit rate 0xf00000000 bytes/second

  divide error: 0000 [#1] SMP
  [ ... ]
  RIP: 0010:[<ffffffffa059035e>]  [<ffffffffa059035e>]
  nft_limit_pkt_bytes_eval+0x2e/0xa0 [nft_limit]
  Call Trace:
  [<ffffffffa05721bb>] nft_do_chain+0xfb/0x4e0 [nf_tables]
  [<ffffffffa044f236>] ? nf_nat_setup_info+0x96/0x480 [nf_nat]
  [<ffffffff81753767>] ? ipt_do_table+0x327/0x610
  [<ffffffffa044f677>] ? __nf_nat_alloc_null_binding+0x57/0x80 [nf_nat]
  [<ffffffffa058b21f>] nft_ipv4_output+0xaf/0xd0 [nf_tables_ipv4]
  [<ffffffff816f4aa2>] nf_iterate+0x62/0x80
  [<ffffffff816f4b33>] nf_hook_slow+0x73/0xd0
  [<ffffffff81703d0d>] __ip_local_out+0xcd/0xe0
  [<ffffffff81701d90>] ? ip_forward_options+0x1b0/0x1b0
  [<ffffffff81703d3c>] ip_local_out+0x1c/0x40

This is because divisor is 64-bit, but we treat it as a 32-bit integer,
then 0xf00000000 becomes zero, i.e. divisor becomes 0.

Signed-off-by: Liping Zhang <liping.zhang@spreadtrum.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>