Set unicode flag on cifs echo request to avoid Mac error

Mac requires the unicode flag to be set for cifs, even for the smb
echo request (which doesn't have strings).

Without this Mac rejects the periodic echo requests (when mounting
with cifs) that we use to check if server is down

Signed-off-by: Steve French <smfrench@gmail.com>
CC: Stable <stable@vger.kernel.org>