Allow "volsize" property to be visible on snapshots of volumes.

Snapshots of volumes store their size separately from their parent.
Register "volsize" as valid for snapshots.

Re-factor the head dataset validation check in get_numeric_property()
so it's performed whenever a property is requested against
a snapshot.  Previously, properties registered as valid for
ZFS_TYPE_SNAPSHOT|ZFS_TYPE_VOLUME would have been returned for snapshots
regardless of whether the snapshot was of a volume or of a filesystem.