netfilter: ipset: Fix race between IPSET_CMD_CREATE and IPSET_CMD_SWAP

netfilter: ipset: Fix race between IPSET_CMD_CREATE and IPSET_CMD_SWAP

Kyle Zeng reported that there is a race between IPSET_CMD_ADD and IPSET_CMD_SWAP
in netfilter/ip_set, which can lead to the invocation of `__ip_set_put` on a wrong
`set`, triggering the `BUG_ON(set->ref == 0);` check in it.

The race is caused by using the wrong reference counter, i.e. the ref counter instead
of ref_netlink.

Reported-by: Kyle Zeng <zengyhkyle@gmail.com>
Tested-by: Kyle Zeng <zengyhkyle@gmail.com>
Signed-off-by: Jozsef Kadlecsik <kadlec@netfilter.org>