Factor out code calling methods of all device template on a scsi_device
out to three helper functions in scsi.c, make scsi_devicelist static to
it and add a r/w semaphore to protect it.

Make scsi_host_list and scsi_host_hn_list static to hosts.c and remove
the never used scsi_host_tmpl_list (we only add to it and remove from
it but never traverse it)