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: NHannes Reinecke <hare@suse.de>
Reviewed-by: NJohannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: NLee Duncan <lduncan@suse.com>
Signed-off-by: NMartin K. Petersen <martin.petersen@oracle.com>