ksmbd: check nt_len to be at least CIFS_ENCPWD_SIZE in ksmbd_decode_ntlmssp_auth_blob

"nt_len - CIFS_ENCPWD_SIZE" is passed directly from
ksmbd_decode_ntlmssp_auth_blob to ksmbd_auth_ntlmv2. Malicious requests
can set nt_len to less than CIFS_ENCPWD_SIZE, which results in a negative
number (or large unsigned value) used for a subsequent memcpy in
ksmbd_auth_ntlvm2 and can cause a panic.

Fixes: e2f34481 ("cifsd: add server-side procedures for SMB3")
Cc: stable@vger.kernel.org
Signed-off-by: William Liu <will@willsroot.io>
Signed-off-by: Hrvoje Mišetić <misetichrvoje@gmail.com>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>