[SCSI] scsi: Add allow_restart sysfs class attribute

This is a resend of a patch I generated in response to an email sent
by Ruben Faelens <parasietje@gmail.com>. His original email to
linux-scsi requested a method in which he could spin down a scsi disk
when not in use and have the kernel automatically spin it back up when
an I/O was generated to the disk. The infrastructure to automatically
spin a disk up has been in the scsi error handler for some time now,
but it is not enabled by default. This patch adds an sd sysfs attribute
which allows userspace to enable this behavior.

Signed-off-by: Brian King <brking@us.ibm.com>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>