configurable block error injection v3

[Christoph Hellwig <hch@lst.de>]

Hi all,

this series adds a new configurable block error injection facility.
We already have a few to inject block errors, but unfortunately most
of them are either not very useful or hard to use, or both:

 - The fail_make_request failure injection point can't distinguish
   different commands, different ranges in the file and can only injection
   plain I/O errors.
 - the should_fail_bio 'dynamic' failure injection has all the same issues
   as fail_make_request
 - dm-error can only fail all command in the table using BLK_STS_IOERR
   and requires setting up a new block device
 - dm-flakey and dm-dust allow all kinds of configurability, but still
   don't have good error selection, no good support for non-read/write
   commands and are limited to the dm table alignment requirements,
   which for zoned devices enforces setting them up for an entire zone.
   They also once again require setting up a stacked block device,
   which is really annoying in harnesses like xfstests

This series adds a new debugfs-based block layer error injection
that allows to configure what operations and ranges the injection
applied to, and what status to return.  It also allows to configure a
failure ratio similar to the xfs errortag injection.

Changes since v2:
 - improve the documentation a bit
 - fix a spelling mistake in a comment

Changes since v1:
 - drop the should_fail_bio removal and cleanup depending on it, as it's
   used by eBPF programs and thus a hidden UABI.
 - as a result split the code out to it's own Kconfig symbol
 - various error handling fixed pointed out by Keith
 - documentation spelling fixes pointed out by Randy

Diffstat:
 Documentation/block/error-injection.rst |   59 ++++++
 Documentation/block/index.rst           |    1 
 block/Kconfig                           |    7 
 block/Makefile                          |    1 
 block/blk-core.c                        |   86 ++++++--
 block/blk-sysfs.c                       |    4 
 block/blk.h                             |   15 +
 block/error-injection.c                 |  308 ++++++++++++++++++++++++++++++++
 block/genhd.c                           |    4 
 include/linux/blkdev.h                  |    6 
 10 files changed, 471 insertions(+), 20 deletions(-)