netfilter: nf_flow_table: set default timeout after successful insertion

commit 110e4872 upstream.

Set up the default timeout for this new entry otherwise the garbage
collector might quickly remove it right after the flowtable insertion.

Fixes: ac2a6666

 ("netfilter: add generic flow table infrastructure")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>