smb: client: fix OOB in smbCalcSize()

Validate @smb->WordCount to avoid reading off the end of @smb and thus
causing the following KASAN splat:

  BUG: KASAN: slab-out-of-bounds in smbCalcSize+0x32/0x40 [cifs]
  Read of size 2 at addr ffff88801c024ec5 by task cifsd/1328

  CPU: 1 PID: 1328 Comm: cifsd Not tainted 6.7.0-rc5 #9
  Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS
  rel-1.16.2-3-gd478f380-rebuilt.opensuse.org 04/01/2014
  Call Trace:
   <TASK>
   dump_stack_lvl+0x4a/0x80
   print_report+0xcf/0x650
   ? srso_alias_return_thunk+0x5/0xfbef5
   ? srso_alias_return_thunk+0x5/0xfbef5
   ? __phys_addr+0x46/0x90
   kasan_report+0xd8/0x110
   ? smbCalcSize+0x32/0x40 [cifs]
   ? smbCalcSize+0x32/0x40 [cifs]
   kasan_check_range+0x105/0x1b0
   smbCalcSize+0x32/0x40 [cifs]
   checkSMB+0x162/0x370 [cifs]
   ? __pfx_checkSMB+0x10/0x10 [cifs]
   cifs_handle_standard+0xbc/0x2f0 [cifs]
   ? srso_alias_return_thunk+0x5/0xfbef5
   cifs_demultiplex_thread+0xed1/0x1360 [cifs]
   ? __pfx_cifs_demultiplex_thread+0x10/0x10 [cifs]
   ? srso_alias_return_thunk+0x5/0xfbef5
   ? lockdep_hardirqs_on_prepare+0x136/0x210
   ? __pfx_lock_release+0x10/0x10
   ? srso_alias_return_thunk+0x5/0xfbef5
   ? mark_held_locks+0x1a/0x90
   ? lockdep_hardirqs_on_prepare+0x136/0x210
   ? srso_alias_return_thunk+0x5/0xfbef5
   ? srso_alias_return_thunk+0x5/0xfbef5
   ? __kthread_parkme+0xce/0xf0
   ? __pfx_cifs_demultiplex_thread+0x10/0x10 [cifs]
   kthread+0x18d/0x1d0
   ? kthread+0xdb/0x1d0
   ? __pfx_kthread+0x10/0x10
   ret_from_fork+0x34/0x60
   ? __pfx_kthread+0x10/0x10
   ret_from_fork_asm+0x1b/0x30
   </TASK>

This fixes CVE-2023-6606.

Reported-by:  <j51569436@gmail.com>
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=218218


Cc: stable@vger.kernel.org
Signed-off-by: Paulo Alcantara (SUSE) <pc@manguebit.com>
Signed-off-by: Steve French <stfrench@microsoft.com>