ksmbd: fix wrong error response status by using set_smb2_rsp_status()

set_smb2_rsp_status() after __process_request() sets the wrong error
status. This patch resets all iov vectors and sets the error status
on clean one.

Fixes: e2b76ab8 ("ksmbd: add support for read compound")
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>