netfilter: ipset: Make sure listing doesn't grab a set which is just being destroyed.
netfilter: ipset: Make sure listing doesn't grab a set which is just being destroyed.

There was a small window when all sets are destroyed and a concurrent
listing of all sets could grab a set which is just being destroyed.

Signed-off-by: Jozsef Kadlecsik <5a7b97a838c5f3992292d1fd9e7ad830cc55ada6@blackhole.kfki.hu>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs