CVE-2026-98446: xfs: don't let hidden_space go negative in xfs_metafile_resv_init

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

xfs: don't let hiddenspace go negative in xfsmetafileresvinit

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 hiddenspace subtraction can underflow, causing xfsdecfdblocks 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 dblocksavail downwards.

Also fix an error in the adjacent comment.