blk_run_queues() places queues onto a local list without locking.  But
interrupt-time activity in scsi_lib.c will replug these queues, which
involves accessing that list which blk_run_queues() is walking.

Net effect: list corruption and an oops.  Very hard to reproduce...

So hold the lock while blk_run_queues() is walking the local list.

The patch was acked by Jens.  It also uninlines a few largeish
functions and adds a couple of WARN_ON()s in code which worried me.