(1) new helper in scsi_scan.c: scsi_forget_host.  it's the counterpart to
    scsi_scan_host.  it shares code with scsi_remove_single_device
    through a common subroutine, scsi_forget_host.
(2) move scsi_get_host_dev/scsi_free_host_dev (again).  Having them in
    scsi_scan.c allows to make scsi_alloc_sdev/scsi_free_sdev static.