That's a tricky one and large part of that stuff is temporary - it will be
replaced as soon as we have gendisks for all block devices and get sane
refcounting for gendisks.
	* we add per-major lists of gendisks; get_gendisk() looks into
appropriate list instead of looking through the list of all gendisks.
	* we allow driver to override that search - it can call
blk_set_probe(major, probe) and then gendisk will call probe(minor).
blk_set_probe(major, NULL) restores the default behaviour.
	* floppy.c switched to use of gendisks; we have one gendisk per
disk and let floppy_find(minor) return the right one.
	Note that final mechanism will involve a similar construction
but floppy.c is actually the least interesting application - places
where it will really play will include stuff like loading the right
high-level driver when we open /dev/hdX, etc.  And it won't be major-based...