(1)  second arg to scsi_queue_next_request() is only ever non-NULL
     inside scsi_lib.c and only used in the first conditional inside
     that function - ripped out of scsi_queue_next_request() and
     put into a new helper scsi_requeue_command().
(2)  Most remaining callers of are in the form
	scsi_put_command(cmd);
	scsi_queue_next_request(q, NULL);
     add a new helper, scsi_next_command() for them.
(2b) many callers of that still contain a repeated codepath, namely
     everything from scsi_release_request except the final kfree.
     New helper __scsi_release_request() for those.
(3)  All remaining callers loop over the devices of a host and call
     scsi_queue_next_request() on them - new helper
     scsi_run_host_queues().
(4)  scsi_queue_next_request() renamed to scsi_run_queue(), second
     arg is gone and it's static to scsi_lib.c now.