ksmbd: validate session id and tree id in compound request

`smb2_get_msg()` in smb2_get_ksmbd_tcon() and smb2_check_user_session()
will always return the first request smb2 header in a compound request.
if `SMB2_TREE_CONNECT_HE` is the first command in compound request, will
return 0, i.e. The tree id check is skipped.
This patch use ksmbd_req_buf_next() to get current command in compound.

Reported-by: zdi-disclosures@trendmicro.com # ZDI-CAN-21506
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>