libbe(3): Descend into children of datasets w/ mountpoint=none

These datasets will generally be canmount=noauto,mountpoint=none (e.g.
zroot/var) but have children that may need to be mounted. Instead of
skipping that segment for no good reason, descend.

Submitted by: Wes Maag
Reported by: Wes Maag
MFC after: 3 days