netfilter: nf_conntrack: fix incorrect logic in nf_conntrack_init_net

in function nf_conntrack_init_net,when nf_conntrack_timeout_init falied,
we should call nf_conntrack_ecache_fini to do rollback.
but the current code calls nf_conntrack_timeout_fini.

Signed-off-by: Gao feng <gaofeng@cn.fujitsu.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>