start-scrub: need a way to scrub just namespace

[NoufalKMuhammed]

On a host we can have multiple namespaces and each can be assigned to different virtual machines. If we want to scrub on a specific namespace, currently there is no way. With the current ndctl start-scrub command, we can scrub on either all nvdimm buses or some specific nvdimm buses, but no way to limit the scrubbing to a particular namespace. If we scrub on all nvdimm buses or some specific set of nvdimm buses, it will take more time, and it can affect performance of other namespaces so to avoid that we first need to ensure that no one else is using them. So the ask here is to add an option in start-scrub command to limit scrub to the namespace(s) specified.

Currently available usage options:

 usage: ndctl start-scrub [<bus-id> <bus-id2> ... <bus-idN>] [<options>]

    -v, --verbose         turn on debug

The ask is to support something similar to the following command: ndctl start-scrub -n namespace0.0