mm/hmm/test: use char dev with struct device to get device node

mm/hmm/test: use char dev with struct device to get device node

HMM selftests use an in-kernel pseudo device to emulate device memory.
The pseudo device registers a major device range for two or four pseudo
device instances.  User space has a script that reads /proc/devices in
order to find the assigned major number, and sends that to mknod(1), once
for each node.

Change this to properly use cdev and struct device APIs.

Delete the /proc/devices parsing from the user-space test script, now that
it is unnecessary.

Also, delete an unused field in struct dmirror_device: devmem.

Link: https://lkml.kernel.org/r/20220826050631.25771-1-mpenttil@redhat.com
Signed-off-by: Mika Penttilä <mpenttil@redhat.com>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Ralph Campbell <rcampbell@nvidia.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>