netfilter: ctnetlink: fix timestamp support for new conntracks

This patch fixes the missing initialization of the start time if
the timestamp support is enabled.

libnetfilter_conntrack/utils# conntrack -E &
libnetfilter_conntrack/utils# ./conntrack_create
tcp      6 109 ESTABLISHED src=1.1.1.1 dst=2.2.2.2 sport=1025 dport=21 packets=0 bytes=0 [UNREPLIED] src=2.2.2.2 dst=1.1.1.1 sport=21 dport=1025 packets=0 bytes=0 mark=0 delta-time=1303296401 use=2

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