cciss: add BUILD_BUG_ON() for catching bad CommandList_struct alignment

The hardware requires 64-bit alignment of commands, so add a build bug
check for that. The recent commit 8a3173de
didn't change the size of the command, but other additions/changes may and
thus break badly at runtime.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>