usb-storage: tag usb_msd_csw as packed struct

usb-storage: tag usb_msd_csw as packed struct

Without this the struct has the wrong size: sizeof() evaluates
to 16 instead of 13.  In most cases the bug is hidden by the
fact that guests submits a buffer which is exactly 13 bytes
long, so the padding added by the compiler is simply ignored.

But sometimes guests submit a larger buffer and expect a short
transfer, which does not work properly with the wrong struct
size.

Cc: vintagepc404@protonmail.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Fixes: a917d384ac0 ("SCSI TCQ support.")
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20210906045523.1259629-1-kraxel@redhat.com>