Make zpool on GELI work again.

Make zpool on GELI work again.

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 r299839, 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 to allow zpool on
full disk encryption work.

Reviewed by:		allanjude
Differential Revision:	https://reviews.freebsd.org/D26333