CVE-2026-98446

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

xfs: don't let hidden space go negative in xfs metafile resv init

LOLLM points out that if the amount of fdblocks that we can reserve for a metadata btree file goes below the space already used by that file, then the hidden space subtraction can underflow, causing xfs dec fdblocks to subtract a huge amount of space. We never want the target to be less than the used sapce, so fix the logic that adjusts dblocks avail downwards.

Also fix an error in the adjacent comment.