CVE-2026-45858: ext4: don't zero the entire extent if EXT4_EXT_DATA_PARTIAL_VALID1

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

ext4: don't zero the entire extent if EXT4EXTDATAPARTIALVALID1

When allocating initialized blocks from a large unwritten extent, or when splitting an unwritten extent during end I/O and converting it to initialized, there is currently a potential issue of stale data if the extent needs to be split in the middle.

0 A B N [UUUUUUUUUUUU] U: unwritten extent [--DDDDDDDD--] D: valid data |<- ->| ----> this range needs to be initialized

ext4splitextent() first try to split this extent at B with EXT4EXTDATAENTIREVALID1 and EXT4EXTMAYZEROOUT flag set, but ext4splitextentat() failed to split this extent due to temporary lack of space. It zeroout B to N and mark the entire extent from 0 to N as written.

0 A B N [WWWWWWWWWWWW] W: written extent [SSDDDDDDDDZZ] Z: zeroed, S: stale data

ext4splitextent() then try to split this extent at A with EXT4EXTDATAVALID2 flag set. This time, it split successfully and left a stale written extent from 0 to A.

0 A B N [WW|WWWWWWWWWW] [SS|DDDDDDDDZZ]

Fix this by pass EXT4EXTDATAPARTIALVALID1 to ext4splitextentat() when splitting at B, don't convert the entire extent to written and left it as unwritten after zeroing out B to N. The remaining work is just like the standard two-part split. ext4splitextent() will pass the EXT4EXTDATAVALID2 flag when it calls ext4splitextentat() for the second time, allowing it to properly handle the split. If the split is successful, it will keep extent from 0 to A as unwritten.