[SCSI] scsi_debug: fix map_region and unmap_region oops

map_region and unmap_region could access to invalid memory area since
they don't check the size boundary.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Acked-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>