qa: resolve occassional "join is not sorted" qa/128 failure

Resolve failure in 128 when nfs stats are activated, from:
"join: [...] is not sorted: nfs3_client_reqs_readdir 7" -
turned out to be "readdir+" sorting non-deterministically
with "readdir".  Canonicalise as readdirplus (like nfsstat
reports it) and all is well.
linux pmda: add missing help text for per-node CPU metrics