CVE-2026-98868: dm-pcache: reject a kset that overruns its segment

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

dm-pcache: reject a kset that overruns its segment

cachereplay(), the writeback worker and the GC worker read a kset of getksetonmediasize() bytes and advance the position by it. A forged keynum makes that size exceed the segment's remaining space, so the advance walks past the segment and trips the cacheposadvance() BUGON.

Reject a kset whose on-media size exceeds cachesegremain() before use.