CVE-2026-45920: ext4: fix dirtyclusters double decrement on fs shutdown

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

ext4: fix dirtyclusters double decrement on fs shutdown

fstests test generic/388 occasionally reproduces a warning in ext4putsuper() associated with the dirty clusters count:

WARNING: CPU: 7 PID: 76064 at fs/ext4/super.c:1324 ext4putsuper+0x48c/0x590 [ext4]

Tracing the failure shows that the warning fires due to an sdirtyclusterscounter value of -1. IOW, this appears to be a spurious decrement as opposed to some sort of leak. Further tracing of the dirty cluster count deltas and an LLM scan of the resulting output identified the cause as a double decrement in the error path between ext4mbmarkdiskspaceused() and the caller ext4mbnewblocks().

First, note that generic/388 is a shutdown vs. fsstress test and so produces a random set of operations and shutdown injections. In the problematic case, the shutdown triggers an error return from the ext4handledirtymetadata() call(s) made from ext4mbmarkcontext(). The changed value is non-zero at this point, so ext4mbmarkdiskspaceused() does not exit after the error bubbles up from ext4mbmarkcontext(). Instead, the former decrements both cluster counters and returns the error up to ext4mbnewblocks(). The latter falls into the !ar->len out path which decrements the dirty clusters counter a second time, creating the inconsistency.

To avoid this problem and simplify ownership of the cluster reservation in this codepath, lift the counter reduction to a single place in the caller. This makes it more clear that ext4mbnewblocks() is responsible for acquiring cluster reservation (via ext4claimfreeclusters()) in the !delalloc case as well as releasing it, regardless of whether it ends up consumed or returned due to failure.