ksmbd: use ksmbd_req_buf_next() in ksmbd_verify_smb_message()

Use ksmbd_req_buf_next() in ksmbd_verify_smb_message().

Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Ralph Boehme <slow@samba.org>
Signed-off-by: Steve French <stfrench@microsoft.com>