netfilter: ipset: prevent uninit-value in hash_ip6_add

syzbot found that we are not validating user input properly
before copying 16 bytes [1].

Using NLA_BINARY in ipaddr_policy[] for IPv6 address is not correct,
since it ensures at most 16 bytes were provided.

We should instead make sure user provided exactly 16 bytes.

In old kernels (before v4.20), fix would be to remove the NLA_BINARY,
since NLA_POLICY_EXACT_LEN() was not yet available.

[1]
BUG: KMSAN: uninit-value in hash_ip6_add+0x1cba/0x3a50 net/netfilter/ipset/ip_set_hash_gen.h:892
CPU: 1 PID: 11611 Comm: syz-executor.0 Not tainted 5.10.0-rc4-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x21c/0x280 lib/dump_stack.c:118
 kmsan_report+0xf7/0x1e0 mm/kmsan/kmsan_report.c:118
 __msan_warning+0x5f/0xa0 mm/kmsan/kmsan_instr.c:197
 hash_ip6_add+0x1cba/0x3a50 net/netfilter/ipset/ip_set_hash_gen.h:892
 hash_ip6_uadt+0x976/0xbd0 net...