Fix diskstats exclude flags (#2487)
Fix diskstats exclude flags (#2487)

Correctly handle the new `collector.diskstats.device-exclude` flag to
avoid errors when using the old `collector.diskstats.ignored-devices`
flag.

Fixes: https://github.com/prometheus/node_exporter/issues/2486

Signed-off-by: Ben Kochie <9bbf917dc63fbaa40c67921332d26cf16a50db75@gmail.com>
prometheus/node_exporter,prometheus/node_exporter,prometheus/node_exporter