CVE-2026-98831: of/overlay: don't leak fragment references when changeset init fails

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

of/overlay: don't leak fragment references when changeset init fails

initoverlaychangeset() stores the number of initialized fragments in ovcs->count only on full success. When findtarget() or the symbols lookup fails partway through, the target and overlay references taken for the fragments initialized so far are never dropped: freeoverlaychangeset() bounds its cleanup loop by ovcs->count, which is still 0.

Store the running count on the error path so freeoverlaychangeset() puts whatever was set up. The store is guarded by ovcs->fragments because on the allocation-failure path cnt still holds the counting pass total while there is no fragments array; everywhere else cnt only counts fully-initialized fragments.