net/sched: act_ct: set 'net' pointer when creating new nf_flow_table

Following patches in series use the pointer to access flow table offload
debug variables.

Signed-off-by: Vlad Buslov <vladbu@nvidia.com>
Signed-off-by: Oz Shlomo <ozsh@nvidia.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>