Enable NBD

This would enable nbd support in FIO which can be helpful for some tests (https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=966222).

--- below is the old - no more applicable text (but without it the interim comments don't make much sense anymore) ---

Hi, I was looking at a bunch of failures in Ubuntu due to the sync of fio-3.21. One was s390x being broken which is fixed.

The other issue I had doesn't affect Debian as it is a memory increase that can break autopkgtests. But the test needs isolation-machine which reduces the architectures it runs on anyway. You are lucky that the VM based test environments in ci.debian work fine. On the Ubuntu side the ppc64el test is sizes in a way to OOM due to that - which will make me need to flag that as needing more memory.

But at the end of that debugging I had a ready-to-use 3.23 merge that I hereby wanted to provide to Debian. It will fix that mentioned s390x issue for you and much more.

In addition I have taken the chance to enable nbd support for this bug.

I'll also attach a sbuild build log in Debian unstable so that you can easily quick check this. fio_3.23-1_amd64.build

P.S.: I have pushed pristine-tar !5 (closed) and upstream !4 (closed) branch merges as well witch match this MP and together should work fine.