netfilter: iptables: Fix null-ptr-deref in iptable_nat_table_init().

In the Linux kernel, the following vulnerability has been resolved:

netfilter: iptables: Fix null-ptr-deref in iptable_nat_table_init().

We had a report that iptables-restore sometimes triggered null-ptr-deref
at boot time. [0]

The problem is that iptable_nat_table_init() is exposed to user space
before the kernel fully initialises netns.

In the small race window, a user could call iptable_nat_table_init()
that accesses net_generic(net, iptable_nat_net_id), which is available
only after registering iptable_nat_net_ops.

Let's call register_pernet_subsys() before xt_register_template().

[0]:
bpfilter: Loaded bpfilter_umh pid 11702
Started bpfilter
BUG: kernel NULL pointer dereference, address: 0000000000000013
PF: supervisor write access in kernel mode
PF: error_code(0x0002) - not-present page
PGD 0 P4D 0
PREEMPT SMP NOPTI
CPU: 2 PID: 11879 Comm: iptables-restor Not tainted 6.1.92-99.174.amzn2023.x86_64 #1
Hardware name: Amazon EC2 c6i.4xlarge/, BIOS 1.0 10/16/2017
RIP: 0010:iptable_nat_table_init (net/ipv4/netfilter/iptable_nat.c:87 net/ipv4/netfilter/iptable_nat.c:121) iptable_nat
Code: 10 4c 89 f6 48 89 ef e8 0b 19 bb ff 41 89 c4 85 c0 75 38 41 83 c7 01 49 83 c6 28 41 83 ff 04 75 dc 48 8b 44 24 08 48 8b 0c 24 <48> 89 08 4c 89 ef e8 a2 3b a2 cf 48 83 c4 10 44 89 e0 5b 5d 41 5c
RSP: 0018:ffffbef902843cd0 EFLAGS: 00010246
RAX: 0000000000000013 RBX: ffff9f4b052caa20 RCX: ffff9f4b20988d80
RDX: 0000000000000000 RSI: 0000000000000064 RDI: ffffffffc04201c0
RBP: ffff9f4b29394000 R08: ffff9f4b07f77258 R09: ffff9f4b07f77240
R10: 0000000000000000 R11: ffff9f4b09635388 R12: 0000000000000000
R13: ffff9f4b1a3c6c00 R14: ffff9f4b20988e20 R15: 0000000000000004
FS: 00007f6284340000(0000) GS:ffff9f51fe280000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000013 CR3: 00000001d10a6005 CR4: 00000000007706e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
PKRU: 55555554
Call Trace:
<TASK>
? show_trace_log_lvl (arch/x86/kernel/dumpstack.c:259)
? show_trace_log_lvl (arch/x86/kernel/dumpstack.c:259)
? xt_find_table_lock (net/netfilter/x_tables.c:1259)
? __die_body.cold (arch/x86/kernel/dumpstack.c:478 arch/x86/kernel/dumpstack.c:420)
? page_fault_oops (arch/x86/mm/fault.c:727)
? exc_page_fault (./arch/x86/include/asm/irqflags.h:40 ./arch/x86/include/asm/irqflags.h:75 arch/x86/mm/fault.c:1470 arch/x86/mm/fault.c:1518)
? asm_exc_page_fault (./arch/x86/include/asm/idtentry.h:570)
? iptable_nat_table_init (net/ipv4/netfilter/iptable_nat.c:87 net/ipv4/netfilter/iptable_nat.c:121) iptable_nat
xt_find_table_lock (net/netfilter/x_tables.c:1259)
xt_request_find_table_lock (net/netfilter/x_tables.c:1287)
get_info (net/ipv4/netfilter/ip_tables.c:965)
? security_capable (security/security.c:809 (discriminator 13))
? ns_capable (kernel/capability.c:376 kernel/capability.c:397)
? do_ipt_get_ctl (net/ipv4/netfilter/ip_tables.c:1656)
? bpfilter_send_req (net/bpfilter/bpfilter_kern.c:52) bpfilter
nf_getsockopt (net/netfilter/nf_sockopt.c:116)
ip_getsockopt (net/ipv4/ip_sockglue.c:1827)
__sys_getsockopt (net/socket.c:2327)
__x64_sys_getsockopt (net/socket.c:2342 net/socket.c:2339 net/socket.c:2339)
do_syscall_64 (arch/x86/entry/common.c:51 arch/x86/entry/common.c:81)
entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:121)
RIP: 0033:0x7f62844685ee
Code: 48 8b 0d 45 28 0f 00 f7 d8 64 89 01 48 83 c8 ff c3 66 2e 0f 1f 84 00 00 00 00 00 90 f3 0f 1e fa 49 89 ca b8 37 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 0a c3 66 0f 1f 84 00 00 00 00 00 48 8b 15 09
RSP: 002b:00007ffd1f83d638 EFLAGS: 00000246 ORIG_RAX: 0000000000000037
RAX: ffffffffffffffda RBX: 00007ffd1f83d680 RCX: 00007f62844685ee
RDX: 0000000000000040 RSI: 0000000000000000 RDI: 0000000000000004
RBP: 0000000000000004 R08: 00007ffd1f83d670 R09: 0000558798ffa2a0
R10: 00007ffd1f83d680 R11: 0000000000000246 R12: 00007ffd1f83e3b2
R13: 00007f6284
---truncated---