Update 'zfs.sh -u' to umount all zfs filesystems

Before it is safe to unload the zfs module stack all mounted
zfs filesystems must be unmounted.  If they are not unmounted,
there will be references held on the modules and the stack cannot
be removed.  To handle this have 'zfs.sh -u' which is used by all
of the test scripts umount all zfs filesystem before attempting
to unload the module stack.

scripts/zfs.sh

@@ -65,6 +65,7 @@ if [ $(id -u) != 0 ]; then
 fi
 
 if [ ${UNLOAD} ]; then
+	umount -t zfs -a
 	unload_modules
 else
 	check_modules || die "${ERROR}"