scsi: iscsi: Release endpoint ID when its freed

We can't release the endpoint ID until all references to the endpoint have
been dropped or it could be allocated while in use. This has us use an idr
instead of looping over all conns to find a free ID and then free the ID
when all references have been dropped instead of when the device is only
deleted.

Link: https://lore.kernel.org/r/20220408001314.5014-4-michael.christie@oracle.com
Tested-by: Manish Rangankar <mrangankar@marvell.com>
Reviewed-by: Lee Duncan <lduncan@suse.com>
Reviewed-by: Chris Leech <cleech@redhat.com>
Reviewed-by: Wu Bo <wubo40@huawei.com>
Signed-off-by: Mike Christie <michael.christie@oracle.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>