xfs_repair: rebuild directory when non-root leafn blocks claim block 0

xfs_repair: rebuild directory when non-root leafn blocks claim block 0

As explained in

71a6af8 Revert "xfs_repair: treat zero da btree pointers as corruption"

a single root LEAFN block can exist in a directory until it grows
further.

This is why, normally, we skip directories with a root marked
XFS_DIR2_LEAFN_MAGIC, as detected by the left-most leaf block being
found at file block 0.

However, if we traversed any level of a btree to get here (as
indicated by da_cursor.active > 0), then a leaf block claiming block
0 indicates corruption, and we should handle it as such, and rebuild
the directory.

This was found by repair repeatedly rebuilding a directory containing a
single leafn block (xfs/495).

Fixes: 67a79e2cc932 ("xfs_repair: treat zero da btree pointers as corruption")
Reported-by: Dave Chinner <david@fromorbit.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
[sandeen: clarify commit log, refer to revert commit]
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>