block: test-iosched: fix spinlock recursion

blk_run_queue() takes the queue spinlock and disabled irqs.
Consider the following callstack:

blk_run_queue
 ->__blk_run_queue
  -> scsi_request_fn
   -> blk_peek_request
    -> __elv_next_request
     -> elevator_dispatch_fn
      -> test_dispatch_requests
       -> test_dispatch_from

test_dispatch_from() will release the test-iosched spinlock
using spin_unlock_irq which will enable interrupts, however,
caller is assuming interrupts are disabled.
An interrupt can occur now and scsi soft-irq may be scheduled
with the following call stack:

scsi_softirq_done
 -> scsi_finish_command
  -> scsi_device_unbusy

scsi_device_unbusy() tries to lock the queue spinlock which was
previously locked when blk_run_queue was called, resulting in a
spinlock recursion.

Change test_dispatch_from() to use the spinlock irq save/restore variants
to prevent enabling the irq in case they were previously disabled.

Change-Id: Icaea4f9ba54771edb0302c6005047fcc5478ce8d
Signed-off-by: Gilad Broner <gbroner@codeaurora.org>