[SCSI] scsi_debug: sweep up sdebug_capacity calculation

sdebug_capacity is calculated at five different places. This add a
helper function to calculate sdebug_capacity to sweep up the
duplicatated code.

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>