simplify gendisk lookup and remove struct block_device aliases v3

[Christoph Hellwig]

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 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

Diffstat:
 b/block/genhd.c           |  183 +++++++--------
 b/drivers/base/Makefile   |    2 
 b/drivers/block/amiflop.c |   98 ++++----
 b/drivers/block/ataflop.c |  135 +++++++----
 b/drivers/block/brd.c     |   39 ---
 b/drivers/block/floppy.c  |  154 ++++++++----
 b/drivers/block/loop.c    |   30 --
 b/drivers/block/swim.c    |   17 -
 b/drivers/block/z2ram.c   |  547 ++++++++++++++++++++++------------------------
 b/drivers/ide/ide-probe.c |   66 -----
 b/drivers/ide/ide-tape.c  |    2 
 b/drivers/md/md.c         |   21 -
 b/drivers/scsi/sd.c       |   19 -
 b/fs/char_dev.c           |   94 +++----
 b/fs/dcache.c             |    1 
 b/fs/internal.h           |    5 
 b/include/linux/genhd.h   |   12 -
 b/include/linux/ide.h     |    3 
 drivers/base/map.c        |  154 ------------
 include/linux/kobj_map.h  |   20 -
 20 files changed, 686 insertions(+), 916 deletions(-)