xfs_repair: don't corrupt a attr fork da3 node when clearing forw/back

xfs_repair: don't corrupt a attr fork da3 node when clearing forw/back

In process_longform_attr, we enforce that the root block of the
attribute index must have both forw or back pointers set to zero.
Unfortunately, the code that nulls out the pointers is not aware that
the root block could be in da3 node format.

This leads to corruption of da3 root node blocks because the functions
that convert attr3 leaf headers to and from the ondisk structures
perform some interpretation of firstused on what they think is an attr1
leaf block.

Found by using xfs/402 to fuzz hdr.info.hdr.forw.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>