ipv6: fix a typo in ip6mr_sk_ioctl()

ipv6: fix a typo in ip6mr_sk_ioctl()

SIOCGETSGCNT_IN6 uses a "struct sioc_sg_req6 buffer".

Unfortunately the blamed commit made hard to ensure type safety.

syzbot reported:

BUG: KASAN: stack-out-of-bounds in ip6mr_ioctl+0xba3/0xcb0 net/ipv6/ip6mr.c:1917
Read of size 16 at addr ffffc900039afb68 by task syz-executor937/5008

CPU: 1 PID: 5008 Comm: syz-executor937 Not tainted 6.4.0-rc6-syzkaller-01304-gc08afcdcf952 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/27/2023
Call Trace:
<TASK>
__dump_stack lib/dump_stack.c:88 [inline]
dump_stack_lvl+0xd9/0x150 lib/dump_stack.c:106
print_address_description.constprop.0+0x2c/0x3c0 mm/kasan/report.c:351
print_report mm/kasan/report.c:462 [inline]
kasan_report+0x11c/0x130 mm/kasan/report.c:572
ip6mr_ioctl+0xba3/0xcb0 net/ipv6/ip6mr.c:1917
rawv6_ioctl+0x4e/0x1e0 net/ipv6/raw.c:1143
sock_ioctl_out net/core/sock.c:4186 [inline]
sk_ioctl+0x151/0x440 net/core/sock.c:4214
inet6_ioctl+0x1b8/0x290 net/ipv6/af_inet6.c:582
sock_do_ioctl+0xcc/0x230 net/socket.c:1189
sock_ioctl+0x1f8/0x680 net/socket.c:1306
vfs_ioctl fs/ioctl.c:51 [inline]
__do_sys_ioctl fs/ioctl.c:870 [inline]
__se_sys_ioctl fs/ioctl.c:856 [inline]
__x64_sys_ioctl+0x197/0x210 fs/ioctl.c:856
do_syscall_x64 arch/x86/entry/common.c:50 [inline]
do_syscall_64+0x39/0xb0 arch/x86/entry/common.c:80
entry_SYSCALL_64_after_hwframe+0x63/0xcd
RIP: 0033:0x7f255849bad9
Code: 28 c3 e8 2a 14 00 00 66 2e 0f 1f 84 00 00 00 00 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 c0 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007ffd06792778 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007f255849bad9
RDX: 0000000000000000 RSI: 00000000000089e1 RDI: 0000000000000003
RBP: 00007f255845fc80 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00007f255845fd10
R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
</TASK>

The buggy address belongs to stack of task syz-executor937/5008
and is located at offset 40 in frame:
sk_ioctl+0x0/0x440 net/core/sock.c:4172

This frame has 2 objects:
[32, 36) 'karg'
[48, 88) 'buffer'

Fixes: e1d001fa5b47 ("net: ioctl: Use kernel memory on protocol ioctl callbacks")
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Willem de Bruijn <willemb@google.com>
Cc: David Ahern <dsahern@kernel.org>
Cc: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: Breno Leitao <leitao@debian.org>
Link: https://lore.kernel.org/r/20230619072740.464528-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>