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 ext4_mb_release()

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->s_discard_list and queue

> s_discard_work. If it is then remounted with nodiscard, the

> EXT4_MOUNT_DISCARD flag is cleared, but the pending s_discard_work is

> neither cancelled nor flushed.

[1] https://lore.kernel.org/r/[email protected]/

[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.