xfs/217: prevent enospc failures on small test devices

I'm running xfstests against a ramdisk, so I'm limited in size of
the test and scratch devices. While there are large enough to hold a
filesystem image with a 2GB log, the way the log changes position in
an image file as the size of the filesystem increases means that the
aggregated disk space of xfs/217 is more than enough to run a 4GB
TEST_DEV out of space and hence fail the test.

To avoid this problem, punch out the image file between every mkfs
iteration so that it only consumes the space needed by each
individual mkfs tests, not an aggregation of them all.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <david@fromorbit.com>

tests/xfs/217

@@ -63,6 +63,10 @@ _do_mkfs()
 		mount -o loop -t xfs $LOOP_DEV $LOOP_MNT
 		echo "test write" > $LOOP_MNT/test
 		umount -d $LOOP_MNT > /dev/null 2>&1
+
+		# punch out the previous blocks so that we keep the amount of
+		# disk space the test requires down to a minimum.
+		$XFS_IO_PROG -f -c "unresvsp 0 16383g" $LOOP_DEV
 	done
 }
 # make large holey file