netfilter: compat: initialize all fields in xt_init

If a non zero value happens to be in xt[NFPROTO_BRIDGE].cur at init
time, the following panic can be caused by running

% ebtables -t broute -F BROUTING

from a 32-bit user level on a 64-bit kernel. This patch replaces
kmalloc_array with kcalloc when allocating xt.

[  474.680846] BUG: unable to handle kernel paging request at 0000000009600920
[  474.687869] PGD 2037006067 P4D 2037006067 PUD 2038938067 PMD 0
[  474.693838] Oops: 0000 [#1] SMP
[  474.697055] CPU: 9 PID: 4662 Comm: ebtables Kdump: loaded Not tainted 4.19.17-11302235.AroraKernelnext.fc18.x86_64 #1
[  474.707721] Hardware name: Supermicro X9DRT/X9DRT, BIOS 3.0 06/28/2013
[  474.714313] RIP: 0010:xt_compat_calc_jump+0x2f/0x63 [x_tables]
[  474.720201] Code: 40 0f b6 ff 55 31 c0 48 6b ff 70 48 03 3d dc 45 00 00 48 89 e5 8b 4f 6c 4c 8b 47 60 ff c9 39 c8 7f 2f 8d 14 08 d1 fa 48 63 fa <41> 39 34 f8 4c 8d 0c fd 00 00 00 00 73 05 8d 42 01 eb e1 76 05 8d
[  474.739023] RSP: 0018:ffffc9000943fc58 EFLAGS: 00010207
[  474.744296] RAX: 0000000000000000 RBX: ffffc90006465000 RCX: 0000000002580249
[  474.751485] RDX: 00000000012c0124 RSI: fffffffff7be17e9 RDI: 00000000012c0124
[  474.758670] RBP: ffffc9000943fc58 R08: 0000000000000000 R09: ffffffff8117cf8f
[  474.765855] R10: ffffc90006477000 R11: 0000000000000000 R12: 0000000000000001
[  474.773048] R13: 0000000000000000 R14: ffffc9000943fcb8 R15: ffffc9000943fcb8
[  474.780234] FS:  0000000000000000(0000) GS:ffff88a03f840000(0063) knlGS:00000000f7ac7700
[  474.788612] CS:  0010 DS: 002b ES: 002b CR0: 0000000080050033
[  474.794632] CR2: 0000000009600920 CR3: 0000002037422006 CR4: 00000000000606e0
[  474.802052] Call Trace:
[  474.804789]  compat_do_replace+0x1fb/0x2a3 [ebtables]
[  474.810105]  compat_do_ebt_set_ctl+0x69/0xe6 [ebtables]
[  474.815605]  ? try_module_get+0x37/0x42
[  474.819716]  compat_nf_setsockopt+0x4f/0x6d
[  474.824172]  compat_ip_setsockopt+0x7e/0x8c
[  474.828641]  compat_raw_setsockopt+0x16/0x3a
[  474.833220]  compat_sock_common_setsockopt+0x1d/0x24
[  474.838458]  __compat_sys_setsockopt+0x17e/0x1b1
[  474.843343]  ? __check_object_size+0x76/0x19a
[  474.847960]  __ia32_compat_sys_socketcall+0x1cb/0x25b
[  474.853276]  do_fast_syscall_32+0xaf/0xf6
[  474.857548]  entry_SYSENTER_compat+0x6b/0x7a

Signed-off-by: Francesco Ruggeri <fruggeri@arista.com>
Acked-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>