block: cciss: fix information leak to userland

Structure IOCTL_Command_struct is copied to userland with
some padding fields at the end of the struct unitialized.
It leads to leaking of contents of kernel stack memory.

Signed-off-by: Vasiliy Kulikov <segooon@gmail.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>