CVE-2026-89830: f2fs: fix valid block count leak on data block allocation failure

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

f2fs: fix valid block count leak on data block allocation failure

In allocatedatablock(), when allocating a new data block (dn->datablkaddr == NULLADDR), incvalidblockcount() is called first to increment totalvalidblockcount and iblocks. If the subsequent f2fsallocatedatablock() fails, the function returns the error directly without rolling back the already-incremented block counts, causing a permanent leak.

Fix this by calling decvalidblockcount() to undo the increment before returning the error. The condition oldblkaddr == NULLADDR precisely identifies the case where incvalidblockcount() was called.