fix NVMe multipath partition diskstats

Currently the NVMe multipath partition diskstats are not maintained. This is because the functionality to update the diskstats only updates for the whole disk (and not per partition).

As an example:

$ ./fio_read nvme1n1p1 # run traffic on /dev/nvme1n1p1
...
$ more /proc/diskstats | grep nvme1
 259       2 nvme1c1n1 49857 0 400344 768565 0 0 0 0 0 2334 768565 0 0 0 0 0 0
 259       3 nvme1n1 99710 0 800680 1599285 0 0 0 0 0 2346 1599285 0 0 0 0 0 0
 259       5 nvme1n1p1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
 259       4 nvme1c2n1 49853 0 400336 831472 0 0 0 0 0 2315 831472 0 0 0 0 0 0

Notice how all values are 0 for nvme1n1p1.

This series addresses that problem by first updating the gendisk code to create the partition table for hidden disks, as suggested by Keith.

This is done by cloning the partition table from the multipath disk.

Secondly the NVMe multipath functionality to handle the diskstats is modified to handle the gendisk partition, by looking up this partition from the per-path disk partition.

Difference to v3:
- Add block device operation disk changed CB

Difference to v2:
- don't scan per-path disk, and clone from multipath disk instead

John Garry (4):
block: support cloning partitions
block: don't show partition diskstats for GENHD_FL_HIDDEN
nvme-multipath: clone head disk partitions to per-path disks
nvme-multipath: fix diskstats for partitions

 block/genhd.c                 |  3 +-
 block/partitions/core.c       | 43 ++++++++++++++++++
 drivers/nvme/host/multipath.c | 83 ++++++++++++++++++++++++++++++-----
 include/linux/blkdev.h        |  2 +
 4 files changed, 120 insertions(+), 11 deletions(-)

2.43.0