CVE-2026-74590: fsverity: Fix bpf_get_fsverity_digest() dynptr assumptions

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

fsverity: Fix bpfgetfsveritydigest() dynptr assumptions

The BPF verifier and the dynptr abstraction ensure that the memory space referenced by a dynptr remains valid. They do not, however, provide any guarantee that the contents of the memory are stable. kfuncs are expected to remain memory-safe even if concurrent modifications occur.

bpfgetfsveritydigest() didn't follow that: it could crash if arg->digestsize was concurrently modified.

Fix that by using the known-good value hashalg->digestsize instead.

Also widen 'dynptrsz' and 'outdigestsz' to u64 to match the return type of bpfdynptrsize(). It doesn't appear that it can actually be more than INTMAX currently (since bpfdynptrdatarw() excludes file-based pointers), but the correct type might as well be used.