Dave Jones reported that KASan detected out of bounds access in hash:net*
types:

[   23.139532] ==================================================================
[   23.146130] BUG: KASan: out of bounds access in hash_net4_add_cidr+0x1db/0x220 at addr ffff8800d4844b58
[   23.152937] Write of size 4 by task ipset/457
[   23.159742] =============================================================================
[   23.166672] BUG kmalloc-512 (Not tainted): kasan: bad access detected
[   23.173641] -----------------------------------------------------------------------------
[   23.194668] INFO: Allocated in hash_net_create+0x16a/0x470 age=7 cpu=1 pid=456
[   23.201836]  __slab_alloc.constprop.66+0x554/0x620
[   23.208994]  __kmalloc+0x2f2/0x360
[   23.216105]  hash_net_create+0x16a/0x470
[   23.223238]  ip_set_create+0x3e6/0x740
[   23.230343]  nfnetlink_rcv_msg+0x599/0x640
[   23.237454]  netlink_rcv_skb+0x14f/0x190
[   23.244533]  nfnetlink_rcv+0x3f6/0x790
[   23.251579]  netlink_unicast+0x272/0x390
[   23.258573]  netlink_sendmsg+0x5a1/0xa50
[   23.265485]  SYSC_sendto+0x1da/0x2c0
[   23.272364]  SyS_sendto+0xe/0x10
[   23.279168]  entry_SYSCALL_64_fastpath+0x12/0x6f

The bug is fixed in the patch and the testsuite is extended in ipset
to check cidr handling more thoroughly.

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