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

ext4_put_super() associated with the dirty clusters count:

WARNING: CPU: 7 PID: 76064 at fs/ext4/super.c:1324 ext4_put_super+0x48c/0x590 [ext4]

Tracing the failure shows that the warning fires due to an

s_dirtyclusters_counter 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 ext4_mb_mark_diskspace_used() and the caller

ext4_mb_new_blocks().

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

ext4_handle_dirty_metadata() call(s) made from

ext4_mb_mark_context(). The changed value is non-zero at this point,

so ext4_mb_mark_diskspace_used() does not exit after the error

bubbles up from ext4_mb_mark_context(). Instead, the former

decrements both cluster counters and returns the error up to

ext4_mb_new_blocks(). 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

ext4_mb_new_blocks() is responsible for acquiring cluster

reservation (via ext4_claim_free_clusters()) in the !delalloc case

as well as releasing it, regardless of whether it ends up consumed

or returned due to failure.