[SCSI] sd: fix cache flushing on module removal (and individual device removal)

The fix isn't actually in sd: it's in scsi_device_get().  I modified it
to allow devices to be returned in SDEV_CANCEL, but not SDEV_DEL.  This
means that the device_remove_driver, which occurs in device_del() in
scsi_remove_device() after the device has gone into SDEV_CANCEL is now
effective at flushing the cache.

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