simplify gendisk lookup and remove struct block_device aliases v4

Hi all,

this series removes the annoying struct block_device aliases, which can
happen for a bunch of old floppy drivers (and z2ram).  In that case
multiple struct block device instances for different dev_t's can point
to the same gendisk, without being partitions.  The cause for that
is the probe/get callback registered through blk_register_regions.

This series removes blk_register_region entirely, splitting it it into
a simple xarray lookup of registered gendisks, and a probe callback
stored in the major_names array that can be used for modprobe overrides
or creating devices on demands when no gendisk is found.  The old
remapping is gone entirely, and instead the 4 remaining drivers just
register a gendisk for each operating mode.  In case of the two drivers
that have lots of aliases that is done on-demand using the new probe
callback, while for the other two I simply register all at probe time
to keep things simple.

Note that the m68k drivers are compile tested only.

Changes since v3:
 - keep kobj_map for char dev lookup for now, as the testbot found
   some very strange and unexplained regressions, so I'll get back to
   this later separately
 - fix a commit message typo

Changes since v2:
 - fix a wrong variable passed to ERR_PTR in the floppy driver
 - slightly adjust the del_gendisk cleanups to prepare for the next
   series touching this area

Changes since v1:
 - add back a missing kobject_put in the cdev code
 - improve the xarray delete loops