ext4: don't cache extent during splitting extent

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

ext4: don't cache extent during splitting extent

Caching extents during the splitting process is risky, as it may result

in stale extents remaining in the status tree. Moreover, in most cases,

the corresponding extent block entries are likely already cached before

the split happens, making caching here not particularly useful.

Assume we have an unwritten extent, and then DIO writes the first half.

[UUUUUUUUUUUUUUUU] on-disk extent U: unwritten extent

[UUUUUUUUUUUUUUUU] extent status tree

|<- ->| ----> dio write this range

First, when ext4_split_extent_at() splits this extent, it truncates the

existing extent and then inserts a new one. During this process, this

extent status entry may be shrunk, and calls to ext4_find_extent() and

ext4_cache_extents() may occur, which could potentially insert the

truncated range as a hole into the extent status tree. After the split

is completed, this hole is not replaced with the correct status.

[UUUUUUU|UUUUUUUU] on-disk extent U: unwritten extent

[UUUUUUU|HHHHHHHH] extent status tree H: hole

Then, the outer calling functions will not correct this remaining hole

extent either. Finally, if we perform a delayed buffer write on this

latter part, it will re-insert the delayed extent and cause an error in

space accounting.

In adition, if the unwritten extent cache is not shrunk during the

splitting, ext4_cache_extents() also conflicts with existing extents

when caching extents. In the future, we will add checks when caching

extents, which will trigger a warning. Therefore, Do not cache extents

that are being split.