[SCSI] scsi_debug: Convert to use root_device_register() and root_device_unregister()

This patch updates the scsi_debug virtual LLD to use
root_device_register() and root_device_unregister() from
include/linux/device.h instead of device_register() and
device_unregister() respectively within scsi_debug_init() and
scsi_debug_exit() This simply involved converting the static struct
device pseudo_primary into a pointer that is setup by the call to
root_device_register().

This patch also contains the correct IS_ERR() conditional check of
root_device_register() from within scsi_debug_init().

Thanks to Richard Sharpe and Dmitry Torokhov for their help with this
item.

Signed-off-by: Nicholas A. Bellinger <nab@linux-iscsi.org>
Acked-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>