netfilter: nfnetlink_queue: avoid harmless unnitialized variable warnings

Several ARM default configurations give us warnings on recent
compilers about potentially uninitialized variables in the
nfnetlink code in two functions:

net/netfilter/nfnetlink_queue.c: In function 'nfqnl_build_packet_message':
net/netfilter/nfnetlink_queue.c:519:19: warning: 'nfnl_ct' may be used uninitialized in this function [-Wmaybe-uninitialized]
  if (ct && nfnl_ct->build(skb, ct, ctinfo, NFQA_CT, NFQA_CT_INFO) < 0)

Moving the rcu_dereference(nfnl_ct_hook) call outside of the
conditional code avoids the warning without forcing us to
preinitialize the variable.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: a4b4766c ("netfilter: nfnetlink_queue: rename related to nfqueue attaching conntrack info")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>