chg: nil: Reduce the code duplication around getting slabheaders from slabtop

There was a lot of duplicated code around getting the first header that
exists, is active, and matches the version header from the qpzonedb.
Move the duplicate code into a helper function and unify the same
approach for the qpcache too even though the code is much simpler there.
It should come handy when top->header is something more complicated than
a pointer to first slabheader.

Merge branch 'ondrej/refactor-getting-the-first-slabheader-from-slabtop' into 'main'

See merge request isc-projects/bind9!10953