ksmbd: fix wrong UserName check in session_user

ksmbd: fix wrong UserName check in session_user

The offset of UserName is related to the address of security
buffer. To ensure the validaty of UserName, we need to compare name_off
+ name_len with secbuf_len instead of auth_msg_len.

[   27.096243] ==================================================================
[   27.096890] BUG: KASAN: slab-out-of-bounds in smb_strndup_from_utf16+0x188/0x350
[   27.097609] Read of size 2 at addr ffff888005e3b542 by task kworker/0:0/7
...
[   27.099950] Call Trace:
[   27.100194]  <TASK>
[   27.100397]  dump_stack_lvl+0x33/0x50
[   27.100752]  print_report+0xcc/0x620
[   27.102305]  kasan_report+0xae/0xe0
[   27.103072]  kasan_check_range+0x35/0x1b0
[   27.103757]  smb_strndup_from_utf16+0x188/0x350
[   27.105474]  smb2_sess_setup+0xaf8/0x19c0
[   27.107935]  handle_ksmbd_work+0x274/0x810
[   27.108315]  process_one_work+0x419/0x760
[   27.108689]  worker_thread+0x2a2/0x6f0
[   27.109385]  kthread+0x160/0x190
[   27.110129]  ret_from_fork+0x1f/0x30
[   27.110454]  </TASK>

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