xfs/073: avoid large recursive diff

xfs/073: avoid large recursive diff

xfs/073 has been failing for me for a while on most of my test setups
with:

diff: memory exhausted

from the large recursive diff it does.  Replace that with a pipe using
md5sum to reduce the memory usage.

Based on a snipplet from Darrick Wong.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Zorro Lang <zlang@kernel.org>