if a datastore or root is not used directly on the pool dir
(e.g. the installer creates 2 sub datasets ROOT/pbs-1), info in
/proc/self/mountinfo returns not the pool, but the path to the
dataset, which has no iostats itself in /proc/spl/kstat/zfs/
but only the pool itself
so instead of not gathering data at all, gather the info from the
underlying pool instead. if one has multiple datastores on the same
pool those rrd stats will be the same for all those datastores now
(instead of empty) similar to 'normal' directories
Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>
368f4c5416