netfilter: nf_tables: check tprot_set first when we use xt.thoff

pkt->xt.thoff is not always set properly, but we use it without any check.
For payload expr, it will cause wrong results. For nftrace, we may notify
the wrong network or transport header to the user space, furthermore,
input the following nft rules, warning message will be printed out:
  # nft add rule arp filter output meta nftrace set 1

  WARNING: CPU: 0 PID: 13428 at net/netfilter/nf_tables_trace.c:263
  nft_trace_notify+0x4a3/0x5e0 [nf_tables]
  Call Trace:
  [<ffffffff813d58ae>] dump_stack+0x63/0x85
  [<ffffffff810a4c0b>] __warn+0xcb/0xf0
  [<ffffffff810a4d3d>] warn_slowpath_null+0x1d/0x20
  [<ffffffffa0589703>] nft_trace_notify+0x4a3/0x5e0 [nf_tables]
  [ ... ]
  [<ffffffffa05690a8>] nft_do_chain_arp+0x78/0x90 [nf_tables_arp]
  [<ffffffff816f4aa2>] nf_iterate+0x62/0x80
  [<ffffffff816f4b33>] nf_hook_slow+0x73/0xd0
  [<ffffffff81732bbf>] arp_xmit+0x8f/0xb0
  [ ... ]
  [<ffffffff81732d36>] arp_solicit+0x106/0x2c0

So before we use pkt->xt.thoff, check the tprot_set first.

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