netfilter: nf_tables_trace: fix endiness when dump chain policy

NFTA_TRACE_POLICY attribute is big endian, but we forget to call
htonl to convert it. Fortunately, this attribute is parsed as big
endian in libnftnl.

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