[SCSI] scsi_debug: remove unnecessary sdebug_store_size

sdebug_store_size doesn't need to be static global. It's used at
startup only.

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>