After OpenZFS import, zpool auto import behavior was moved to an explicit "zpool import -a", and the zpool rc.d script was added as a prerequisite of zvol.
However, in rS299839, zvol was added as a prerequisite of dumpon, making it to start very early and before all 'disks' providers. At this time, dumping on a zvol is not supported, so remove this requirement and make zpool depend on disks instead.