[SCSI] allow displaying and setting of cache type via sysfs

I think I promised to do this two years ago

This patch adds a scsi_disk class with the cache type and FUA
parameters, so user land application can easily obtain them without
having to parse dmesg.  It also allows setting the cache type (use with
care...)

This patch is a bit dangerous because I've replaced the disk kref with a
class device reference ...

Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>