netfilter: nft_chain_nat: inet family is missing module ownership

Set owner to THIS_MODULE, otherwise the nft_chain_nat module might be
removed while there are still inet/nat chains in place.

[  117.942096] BUG: unable to handle page fault for address: ffffffffa0d5e040
[  117.942101] #PF: supervisor read access in kernel mode
[  117.942103] #PF: error_code(0x0000) - not-present page
[  117.942106] PGD 200c067 P4D 200c067 PUD 200d063 PMD 3dc909067 PTE 0
[  117.942113] Oops: 0000 [#1] PREEMPT SMP PTI
[  117.942118] CPU: 3 PID: 27 Comm: kworker/3:0 Not tainted 5.6.0-rc3+ #348
[  117.942133] Workqueue: events nf_tables_trans_destroy_work [nf_tables]
[  117.942145] RIP: 0010:nf_tables_chain_destroy.isra.0+0x94/0x15a [nf_tables]
[  117.942149] Code: f6 45 54 01 0f 84 d1 00 00 00 80 3b 05 74 44 48 8b 75 e8 48 c7 c7 72 be de a0 e8 56 e6 2d e0 48 8b 45 e8 48 c7 c7 7f be de a0 <48> 8b 30 e8 43 e6 2d e0 48 8b 45 e8 48 8b 40 10 48 85 c0 74 5b 8b
[  117.942152] RSP: 0018:ffffc9000015be10 EFLAGS: 00010292
[  117.942155] RAX: ffffffffa0d5e040 RBX: ffff88840be87fc2 RCX: 0000000000000007
[  117.942158] RDX: 0000000000000007 RSI: 0000000000000086 RDI: ffffffffa0debe7f
[  117.942160] RBP: ffff888403b54b50 R08: 0000000000001482 R09: 0000000000000004
[  117.942162] R10: 0000000000000000 R11: 0000000000000001 R12: ffff8883eda7e540
[  117.942164] R13: dead000000000122 R14: dead000000000100 R15: ffff888403b3db80
[  117.942167] FS:  0000000000000000(0000) GS:ffff88840e4c0000(0000) knlGS:0000000000000000
[  117.942169] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  117.942172] CR2: ffffffffa0d5e040 CR3: 00000003e4c52002 CR4: 00000000001606e0
[  117.942174] Call Trace:
[  117.942188]  nf_tables_trans_destroy_work.cold+0xd/0x12 [nf_tables]
[  117.942196]  process_one_work+0x1d6/0x3b0
[  117.942200]  worker_thread+0x45/0x3c0
[  117.942203]  ? process_one_work+0x3b0/0x3b0
[  117.942210]  kthread+0x112/0x130
[  117.942214]  ? kthread_create_worker_on_cpu+0x40/0x40
[  117.942221]  ret_from_fork+0x35/0x40

nf_tables_chain_destroy() crashes on module_put() because the module is
gone.

Fixes: d164385e ("netfilter: nat: add inet family nat support")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>