[SCSI] scsi_debug: simplify creation and destruction of driver attribute files

Instead of repeatedly calling driver_create_file() to create driver
attribute files, This achieves the same thing by constructing an array
of driver_attribute and setting it to bus_type->drv_groups.

This change simplifies both creation and destruction of the attribute
files, and also removes sparse warning caused by driver_attributes which
are unnecessarily declared as global.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Acked-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>