ksmbd: validate session id and tree id in compound request

In the Linux kernel, the following vulnerability has been resolved: 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.