xfs_metadump: zap stale data in DIR2_LEAF1 dirs

xfs_metadump: zap stale data in DIR2_LEAF1 dirs

xfs_metadump attempts to zero out unused regions of metadata
blocks to prevent data leaks when sharing metadata images.

However, Stefan Ring reported a significant number of leaked
strings when dumping his 1T filesystem.  Based on a reduced
metadata set, I was able to identify "leaf" directories
(with XFS_DIR2_LEAF1_MAGIC magic) as the primary culprit;
the region between the end of the entries array and the start
of the bests array was not getting zeroed out.  This patch
seems to remedy that problem.

Reported-by: Stefan Ring <stefanrin@gmail.com>
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>