The following comes from James Bottemley, as he updated the SCSI layer
to use the generic tagging.

Basically just put blk_queue_tag_request() into ll_rw_blk.c and name it
something more sane, blk_queue_find_tag(). In addition, remove REQ_CMD
requirement in blk_queue_start_tag() -- this is actually IDE specific
(we can only tag read/write requests there...), SCSI actually requires
everything to be tagged once enabled. It's replaced with a safety check
for an already tagged request.