netfilter: nf_flow_table: do not remove offload when other netns's interface is down

When interface is down, offload cleanup function(nf_flow_table_do_cleanup)
is called and that checks whether interface index of offload and
index of link down interface is same. but only interface index checking
is not enough because flowtable is not pernet list.
So that, if other netns's interface that has index is same with offload
is down, that offload will be removed.
This patch adds netns checking code to the offload cleanup routine.

Fixes: 59c466dd ("netfilter: nf_flow_table: add a new flow state for tearing down offloading")
Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>