dns_rdataslab_merge(): use dns_rdata_compare() instead of compare_rdata()

dns_rdataslab_merge(): use dns_rdata_compare() instead of compare_rdata()

compare_rdata() was meant to be used as a qsort() callback.  Meanwhile,
dns_rdataslab_merge() calls compare_rdata() for a pair of dns_rdata_t
structures rather than a pair of struct xrdata structures, which is
harmless, but triggers an ubsan warning:

    rdataslab.c:84:33: runtime error: member access within address <address> with insufficient space for an object of type 'const struct xrdata'

Use dns_rdata_compare() instead of compare_rdata() to prevent the
warning from being triggered.

(cherry picked from commit 9bc6ba0be9447c66d8f2b055a696ff12842c7863)