For various reasons (e.g. the .my_devices lockdown) it's undesirable
to have scsi_device_{get,put} blocking.  Change .access_count to an
atomic_t to avoid the driver model r/w semaphore.

Sideeffects:

 - possible module reference leak in scsi_device_get when get_device
   fails fixes.
 - the access_count attribute is gone.  We already have this exposed
   to userspace in too many places given that this whole second
   refcount will go away once Al fixes the block layer.