netfilter: nf_tables: fix oops when inserting an element into a verdict map

netfilter: nf_tables: fix oops when inserting an element into a verdict map

Dalegaard says:
 The following ruleset, when loaded with 'nft -f bad.txt'
 ----snip----
 flush ruleset
 table ip inlinenat {
   map sourcemap {
     type ipv4_addr : verdict;
   }

   chain postrouting {
     ip saddr vmap @sourcemap accept
   }
 }
 add chain inlinenat test
 add element inlinenat sourcemap { 100.123.10.2 : jump test }
 ----snip----

 results in a kernel oops:
 BUG: unable to handle kernel paging request at 0000000000001344
 IP: [<ffffffffa07bf704>] nf_tables_check_loops+0x114/0x1f0 [nf_tables]
 [...]
 Call Trace:
  [<ffffffffa07c2aae>] ? nft_data_init+0x13e/0x1a0 [nf_tables]
  [<ffffffffa07c1950>] nft_validate_register_store+0x60/0xb0 [nf_tables]
  [<ffffffffa07c74b5>] nft_add_set_elem+0x545/0x5e0 [nf_tables]
  [<ffffffffa07bfdd0>] ? nft_table_lookup+0x30/0x60 [nf_tables]
  [<ffffffff8132c630>] ? nla_strcmp+0x40/0x50
  [<ffffffffa07c766e>] nf_tables_newsetelem+0x11e/0x210 [nf_tables]
  [<ffffffff8132c400>] ? nla_validate+0x60/0x80
  [<ffffffffa030d9b4>] nfnetlink_rcv+0x354/0x5a7 [nfnetlink]

Because we forget to fill the net pointer in bind_ctx, so dereferencing
it may cause kernel crash.

Reported-by: Dalegaard <dalegaard@gmail.com>
Signed-off-by: Liping Zhang <zlpnobody@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>