Add proper refcounting to zfs graphdriver

Fixes issues with layer remounting (e.g. a running container which then
has `docker cp` used to copy files in or out) by applying the same
refcounting implementation that exists in other graphdrivers like
overlay and aufs.

Docker-DCO-1.1-Signed-off-by: Phil Estes <estesp@linux.vnet.ibm.com> (github: estesp)