scsi: sd: Rely on the driver core for asynchronous probing

As explained during the 2018 LSF/MM session about increasing SCSI disk
probing concurrency, the problems with the current probing approach are as
follows:

- The driver core is unaware of asynchronous SCSI LUN probing.
  wait_for_device_probe() waits for all asynchronous probes except
  asynchronous SCSI disk probes.

- There is unnecessary serialization between sd_probe() and sd_remove().
  This can lead to a deadlock.

Hence this patch that modifies the sd driver such that it uses the driver
core framework for asynchronous probing. The async domains and
get_device()/put_device() pairs that became superfluous due to this change
are removed.

This patch does not affect the time needed for loading the scsi_debug
kernel module with parameters delay=0 and max_luns=256.

This patch depends on commit ef0ff683 ("driver core: Probe devices
asynchronously instead of the driver") that went upstream in kernel version
v5.1-rc1.

Cc: Lee Duncan <lduncan@suse.com>
Cc: Hannes Reinecke <hare@suse.com>
Cc: Luis Chamberlain <mcgrof@kernel.org>
Cc: Johannes Thumshirn <jthumshirn@suse.de>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>