dnsdist: Increment the "dyn blocked" counter for eBPF blocks as well

dnsdist: Increment the "dyn blocked" counter for eBPF blocks as well

Regular, userspace blocks increment the "dyn blocked" counter for every
dropped query. The eBPF blocks are executed in kernelspace and thus do
not increment that counter at all, which makes it challenging for
reporting to do its job. On the other hand we want our eBPF code to
be as efficient as possible since it is used when performance really
matters.
This commit updates the counter when a eBPF dynamic block is removed,
which is a compromise between the performance impact and a slight
reporting delay.