This changes the `modprobe nbd` invocation to be non-fatal on failure,
since it might very well fail on reasonable setups. `modprobe` fails if
it cannot find a module in `/lib/modules`, even if it could reasonably
well figure out whether a module is already loaded. The reason is that
it needs the metadata from the module file to find the required modules
parameters.
If you have `nbd` already loaded but not mapped in `/lib/modules`, the
current call will cause test failures, even though the test would run
smoothly.
Fix this by never requiring `modprobe nbd` to succeed, but instead rely
on the tests failing if accessing `nbd` fails.
15e4d0d3a1