ksmbd: validate command payload size

->StructureSize2 indicates command payload size. ksmbd should validate
this size with rfc1002 length before accessing it.
This patch remove unneeded check and add the validation for this.

[    8.912583] BUG: KASAN: slab-out-of-bounds in ksmbd_smb2_check_message+0x12a/0xc50
[    8.913051] Read of size 2 at addr ffff88800ac7d92c by task kworker/0:0/7
...
[    8.914967] Call Trace:
[    8.915126]  <TASK>
[    8.915267]  dump_stack_lvl+0x33/0x50
[    8.915506]  print_report+0xcc/0x620
[    8.916558]  kasan_report+0xae/0xe0
[    8.917080]  kasan_check_range+0x35/0x1b0
[    8.917334]  ksmbd_smb2_check_message+0x12a/0xc50
[    8.917935]  ksmbd_verify_smb_message+0xae/0xd0
[    8.918223]  handle_ksmbd_work+0x192/0x820
[    8.918478]  process_one_work+0x419/0x760
[    8.918727]  worker_thread+0x2a2/0x6f0
[    8.919222]  kthread+0x187/0x1d0
[    8.919723]  ret_from_fork+0x1f/0x30
[    8.919954]  </TASK>

Cc: stable@vger.kernel.org
Reported-by: Chih-Yen Chang <cc85nod@gmail.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>