Instead of comparing buffer pointers, compare buffer block numbers and
don't keep buff

SGI-PV: 983564

SGI-Modid: xfs-linux-melb:xfs-kern:31346a
Signed-off-by: Barry Naujok <bnaujok@sgi.com>
Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>