syzbot reports one unsafe call to copy_from_sockptr() [1]

Use copy_safe_from_sockptr() instead.

[1]

 BUG: KASAN: slab-out-of-bounds in copy_from_sockptr_offset include/linux/sockptr.h:49 [inline]
 BUG: KASAN: slab-out-of-bounds in copy_from_sockptr include/linux/sockptr.h:55 [inline]
 BUG: KASAN: slab-out-of-bounds in data_sock_setsockopt+0x46c/0x4cc drivers/isdn/mISDN/socket.c:417
Read of size 4 at addr ffff0000c6d54083 by task syz-executor406/6167

CPU: 1 PID: 6167 Comm: syz-executor406 Not tainted 6.8.0-rc7-syzkaller-g707081b61156 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 03/27/2024
Call trace:
  dump_backtrace+0x1b8/0x1e4 arch/arm64/kernel/stacktrace.c:291
  show_stack+0x2c/0x3c arch/arm64/kernel/stacktrace.c:298
  __dump_stack lib/dump_stack.c:88 [inline]
  dump_stack_lvl+0xd0/0x124 lib/dump_stack.c:106
  print_address_description mm/kasan/report.c:377 [inline]
  print_report+0x178/0x518 mm/kasan/report.c:488
  kasan_report+0xd8/0x138 mm/kasan/report.c:601
  __asan_report_load_n_noabort+0x1c/0x28 mm/kasan/report_generic.c:391
  copy_from_sockptr_offset include/linux/sockptr.h:49 [inline]
  copy_from_sockptr include/linux/sockptr.h:55 [inline]
  data_sock_setsockopt+0x46c/0x4cc drivers/isdn/mISDN/socket.c:417
  do_sock_setsockopt+0x2a0/0x4e0 net/socket.c:2311
  __sys_setsockopt+0x128/0x1a8 net/socket.c:2334
  __do_sys_setsockopt net/socket.c:2343 [inline]
  __se_sys_setsockopt net/socket.c:2340 [inline]
  __arm64_sys_setsockopt+0xb8/0xd4 net/socket.c:2340
  __invoke_syscall arch/arm64/kernel/syscall.c:34 [inline]
  invoke_syscall+0x98/0x2b8 arch/arm64/kernel/syscall.c:48
  el0_svc_common+0x130/0x23c arch/arm64/kernel/syscall.c:133
  do_el0_svc+0x48/0x58 arch/arm64/kernel/syscall.c:152
  el0_svc+0x54/0x168 arch/arm64/kernel/entry-common.c:712
  el0t_64_sync_handler+0x84/0xfc arch/arm64/kernel/entry-common.c:730
  el0t_64_sync+0x190/0x194 arch/arm64/kernel/entry.S:598

Fixes: 1b2b03f8 ("Add mISDN core files")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
Cc: Karsten Keil <isdn@linux-pingi.de>
Link: https://lore.kernel.org/r/20240408082845.3957374-3-edumazet@google.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>