[SCSI] sd: Fix refcounting

Currently the driver takes a reference only for requests coming by way
of the gendisk, not for requests coming by way of the struct device or
struct scsi_device.  Such requests can arrive in the rescan, flush,
and shutdown pathways.

The patch also makes the scsi_disk keep a reference to the underlying
scsi_device, and it erases the scsi_device's pointer to the scsi_disk
when the scsi_device is removed (since the pointer should no longer be
used).

This resolves Bugzilla entry #5237.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>