netfilter: nf_tables: minor tracing cleanups

The tracing code is squeezed between multiple related parts of the
evaluation code, move it out. Also add an inline wrapper for the
reoccuring test for skb->nf_trace.

Small code savings in nft_do_chain():

  nft_trace_packet          | -137
  nft_do_chain              |   -8
 2 functions changed, 145 bytes removed, diff: -145

net/netfilter/nf_tables_core.c:
  __nft_trace_packet | +137
 1 function changed, 137 bytes added, diff: +137

net/netfilter/nf_tables_core.o:
 3 functions changed, 137 bytes added, 145 bytes removed, diff: -8

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>