When element of verdict map is deleted, the delete routine should
release chain. however, flush element of verdict map routine doesn't
release chain.

test commands:
   %nft add table ip filter
   %nft add chain ip filter c1
   %nft add map ip filter map1 { type ipv4_addr : verdict \; }
   %nft add element ip filter map1 { 1 : jump c1 }
   %nft flush map ip filter map1
   %nft flush ruleset

splat looks like:
[ 4895.170899] kernel BUG at net/netfilter/nf_tables_api.c:1415!
[ 4895.178114] invalid opcode: 0000 [#1] SMP DEBUG_PAGEALLOC KASAN PTI
[ 4895.178880] CPU: 0 PID: 1670 Comm: nft Not tainted 4.18.0+ #55
[ 4895.178880] RIP: 0010:nf_tables_chain_destroy.isra.28+0x39/0x220 [nf_tables]
[ 4895.178880] Code: fc ff df 53 48 89 fb 48 83 c7 50 48 89 fa 48 c1 ea 03 0f b6 04 02 84 c0 74 09 3c 03 7f 05 e8 3e 4c 25 e1 8b 43 50 85 c0 74 02 <0f> 0b 48 89 da 48 b8 00 00 00 00 00 fc ff df 48 c1 ea 03 80 3c 02
[ 4895.228342] RSP: 0018:ffff88010b98f4c0 EFLAGS: 00010202
[ 4895.234841] RAX: 0000000000000001 RBX: ffff8801131c6968 RCX: ffff8801146585b0
[ 4895.234841] RDX: 1ffff10022638d37 RSI: ffff8801191a9348 RDI: ffff8801131c69b8
[ 4895.234841] RBP: ffff8801146585a8 R08: 1ffff1002323526a R09: 0000000000000000
[ 4895.234841] R10: 0000000000000000 R11: 0000000000000000 R12: dead000000000200
[ 4895.234841] R13: dead000000000100 R14: ffffffffa3638af8 R15: dffffc0000000000
[ 4895.234841] FS:  00007f6d188e6700(0000) GS:ffff88011b600000(0000) knlGS:0000000000000000
[ 4895.234841] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 4895.234841] CR2: 00007ffe72b8df88 CR3: 000000010e2d4000 CR4: 00000000001006f0
[ 4895.234841] Call Trace:
[ 4895.234841]  nf_tables_commit+0x2704/0x2c70 [nf_tables]
[ 4895.234841]  ? nfnetlink_rcv_batch+0xa4f/0x11b0 [nfnetlink]
[ 4895.234841]  ? nf_tables_setelem_notify.constprop.48+0x1a0/0x1a0 [nf_tables]
[ 4895.323824]  ? __lock_is_held+0x9d/0x130
[ 4895.323824]  ? kasan_unpoison_shadow+0x30/0x40
[ 4895.333299]  ? kasan_kmalloc+0xa9/0xc0
[ 4895.333299]  ? kmem_cache_alloc_trace+0x2c0/0x310
[ 4895.333299]  ? nfnetlink_rcv_batch+0xa4f/0x11b0 [nfnetlink]
[ 4895.333299]  nfnetlink_rcv_batch+0xdb9/0x11b0 [nfnetlink]
[ 4895.333299]  ? debug_show_all_locks+0x290/0x290
[ 4895.333299]  ? nfnetlink_net_init+0x150/0x150 [nfnetlink]
[ 4895.333299]  ? sched_clock_cpu+0xe5/0x170
[ 4895.333299]  ? sched_clock_local+0xff/0x130
[ 4895.333299]  ? sched_clock_cpu+0xe5/0x170
[ 4895.333299]  ? find_held_lock+0x39/0x1b0
[ 4895.333299]  ? sched_clock_local+0xff/0x130
[ 4895.333299]  ? memset+0x1f/0x40
[ 4895.333299]  ? nla_parse+0x33/0x260
[ 4895.333299]  ? ns_capable_common+0x6e/0x110
[ 4895.333299]  nfnetlink_rcv+0x2c0/0x310 [nfnetlink]
[ ... ]

Fixes: 59105446

 ("netfilter: nf_tables: revisit chain/object refcounting from elements")
Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>