[2.2] vdev_disk: ensure trim errors are returned immediately

[robn]

Motivation and Context

Backporting #16070 for 2.2.

Description

After 08fd5cc, the discard issuing code was organised such that if requesting an async discard or secure erase failed before the IO was issued (that is, calling __blkdev_issue_discard() returned an error), the failed zio would never be executed, resulting in txg_sync hanging forever waiting for IO to finish.

This commit fixes that by immediately executing a failed zio on error. To handle the successful synchronous op case, we fake an async op by, when not using an asynchronous submission method, queuing the successful result zio as part of the discard handler.

Since it was hard to understand the differences between discard and secure erase, and sync and async, across different kernel versions, I've commented and reorganised the code a bit to try and make everything more contained and linear.

How Has This Been Tested?

Compiled and successfully passed zpool_trim test suites on kernels:

• 4.14.336
• 5.10.214
• 6.1.83
• 6.8.2

On 5.10.214, with loopback devices (which have incorrect discard_granularity, see #16068, both zpool trim and autotrim=on woud hang. With this in place, they appear to succeed, and the failures are recorded in /proc/spl/kstat/zfs/xxx/iostats. This is returning to the previous behaviour.

See also testing on #16070, which should all hold here.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.