add a ZONE_RESET_ALL uring cmd v2

Hi Jens,

this series adds a new ZONE_RESET_ALL uring cmd, and cleans up the
io_uring discard support and the block layer zone management code a
little bit in preparation for that.

This command is useful to speed up workloads that need to initialize a
large number (or even a small > 1 number) of disks.

Diffstat:
 block/blk-core.c            |    6 +++-
 block/blk-zoned.c           |   30 ----------------------
 block/ioctl.c               |   60 ++++++++++++++++++++++++++++++++++----------
 include/uapi/linux/blkdev.h |    1 
 4 files changed, 53 insertions(+), 44 deletions(-)