CVE-2026-43065: ext4: always drain queued discard work in ext4_mb_release()

In the Linux kernel, the following vulnerability has been resolved:

ext4: always drain queued discard work in ext4mbrelease()

While reviewing recent ext4 patch[1], Sashiko raised the following concern[2]:

If the filesystem is initially mounted with the discard option, deleting files will populate sbi->sdiscardlist and queue sdiscardwork. If it is then remounted with nodiscard, the EXT4MOUNTDISCARD flag is cleared, but the pending sdiscardwork is neither cancelled nor flushed.

[1] https://lore.kernel.org/r/20260319094545.19291-1-qiang.zhang@linux.dev/ [2] https://sashiko.dev/#/patchset/20260319094545.19291-1-qiang.zhang%40linux.dev

The concern was valid, but it had nothing to do with the patch[1]. One of the problems with Sashiko in its current (early) form is that it will detect pre-existing issues and report it as a problem with the patch that it is reviewing.

In practice, it would be hard to hit deliberately (unless you are a malicious syzkaller fuzzer), since it would involve mounting the file system with -o discard, and then deleting a large number of files, remounting the file system with -o nodiscard, and then immediately unmounting the file system before the queued discard work has a change to drain on its own.

Fix it because it's a real bug, and to avoid Sashiko from raising this concern when analyzing future patches to mballoc.c.