xfs_repair: invalidate dirty dir buffers when we zap a directory

xfs_repair: invalidate dirty dir buffers when we zap a  directory

If we decide to rebuild a directory in phase 6, we need to find and
invalidate all of the old directory buffers so that they don't get
written out, which can trigger write verifier errors when we finish.
This fixes the write verifier errors in phase 7 that can occur via
xfs/382.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>