scsi: Use ida for host number management

Update the SCSI hosts module to use ida to manage its host_no index
instead of an atomic integer. This means that the SCSI host number will
now be reclaimable.

Use the ida "simple" mechanism, since there should be no need for a
separate spin lock current usage. Ida was chosen over idr because the
hosts module already has its own instance and locking mechanisms that
aren't easily changed.

Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Lee Duncan <lduncan@suse.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>