[SCSI] scsi_debug: use list_for_each_entry_safe

This replaces list_for_each_safe and list_entry with
list_for_each_entry_safe.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Acked-by: Douglas Gilbert <dougg@torque.net>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>