netfilter: ctnetlink: using bit to represent the ct event

Otherwise, creating a new conntrack via nfnetlink:
  # conntrack -I -p udp -s 1.1.1.1 -d 2.2.2.2 -t 10 --sport 10 --dport 20

will emit the wrong ct events(where UPDATE should be NEW):
  # conntrack -E
  [UPDATE] udp      17 10 src=1.1.1.1 dst=2.2.2.2 sport=10 dport=20
  [UNREPLIED] src=2.2.2.2 dst=1.1.1.1 sport=20 dport=10 mark=0

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