Set refquota on when fssize provided

When a filesystem of a specific size is requested the the refquota.
This is going to be an artificial limit since zfs uses pooled storage
but that can be helpful when running tests which are designed to fill
the entire filesystem.

Additionally, update the scratch fs creation to first destroy the
previous dataset and then create a new one.

common.rc

@@ -313,9 +313,10 @@ _scratch_mkfs()
     zfs)
 	# SCRATCH_DEV must be of the form 'pool/dataset'
 	/sbin/zfs list -H $SCRATCH_DEV > /dev/null 2>&1
-	if [ "$?" != "0" ]; then
-	    /sbin/zfs create -o mountpoint=legacy $SCRATCH_DEV
+	if [ "$?" = "0" ]; then
+            /sbin/zfs destroy -f $SCRATCH_DEV
 	fi
+	/sbin/zfs create -o mountpoint=legacy $SCRATCH_DEV
 	;;
     *)
 	/sbin/mkfs -t $FSTYP -- $MKFS_OPTIONS $* $SCRATCH_DEV
@@ -343,12 +344,12 @@ _scratch_mkfs_sized()
 	/sbin/mkfs.$FSTYP $MKFS_OPTIONS $SCRATCH_DEV -b $fssize
 	;;
     zfs)
-	# No size required due to the nature of pooled storage
 	# SCRATCH_DEV must be of the form 'pool/dataset'
 	/sbin/zfs list -H $SCRATCH_DEV > /dev/null 2>&1
-	if [ "$?" != "0" ]; then
-	    /sbin/zfs create -o mountpoint=legacy $SCRATCH_DEV
+	if [ "$?" = "0" ]; then
+            /sbin/zfs destroy -f $SCRATCH_DEV
 	fi
+	/sbin/zfs create -o mountpoint=legacy -o refquota=$fssize $SCRATCH_DEV
 	;;
     *)
 	_notrun "Filesystem $FSTYP not supported in _scratch_mkfs_sized"