Make the "volsize" property visible on snapshots of volumes.

The volsize property corresponds to "size" as stored in a volume's
ZVOL_ZAP_OBJ object.  Since this value is actually physically part of the
snapshot, its corresponding property should be displayed by "zfs get".
This patch allows "zfs get volsize pool/vol@snap" to display its value.

This came about as during the discussion of #2131 but AFAICT, is not related to its root problem that a ZVOL's ARC cache is dropped when the ZVOL is closed.