netfilter: ipset: Fix data race between add and dump in all hash types

netfilter: ipset: Fix data race between add and dump in all hash types

When adding a new entry to the next position in the existing hash bucket,
the position index was incremented too early and parallel dump could
read it before the entry was populated with the value. Move the setting
of the position index after populating the entry.

v2: Position counting fixed, noticed by Florian Westphal.

Fixes: 18f84d41d34f ("netfilter: ipset: Introduce RCU locking in hash:* types")
Reported-by: syzbot+786c889f046e8b003ca6@syzkaller.appspotmail.com
Reported-by: syzbot+1da17e4b41d795df059e@syzkaller.appspotmail.com
Reported-by: syzbot+421c5f3ff8e9493084d9@syzkaller.appspotmail.com
Signed-off-by: Jozsef Kadlecsik <kadlec@netfilter.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>