xfs/014: replace df with $DF_PROG

I've seen xfs/014 fails as

[root@dhcp-66-86-3 xfstests]# diff -u tests/xfs/014.out /var/lib/xfstests/results//xfs/014.out.bad

tests/xfs/014

@@ -88,7 +88,7 @@ _consume_free_space()
 	dir=$1
 
 	# allocate all but 10MB of available space
-	freesp=`df -m $dir | awk '/^\// { print $4 - 10 }'`
+	freesp=`$DF_PROG -m $dir | $AWK_PROG '/^\// { print $5 - 10 }'`
 	$XFS_IO_PROG -f -c "falloc 0 ${freesp}M" $dir/spc
 }