xfs: strengthen attr leaf block freemap checking

xfs: strengthen attr leaf block freemap checking

Check for erroneous overlapping freemap regions and collisions between
freemap regions and the xattr leaf entry array.

Note that we must explicitly zero out the extra freemaps in
xfs_attr3_leaf_compact so that the in-memory buffer has a correctly
initialized freemap array to satisfy the new verification code, even if
subsequent code changes the contents before unlocking the buffer.

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