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

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

This reverts commit 67a79e2cc9320aaf269cd00e9c8d16892931886d.

A root LEAFN block can exist in a directory. When we convert from
leaf format (LEAF1 - internal free list) to node format (LEAFN -
external free list) the only change to the single root leaf block is
that it's magic number is changed from LEAF1 to LEAFN.

We don't actually end up with DA nodes in the tree until the LEAFN
node is split, and that requires a couple more dirents to be added
to the directory to fill the LEAFN block up completely. Then it will
split and create a DA node root block pointing to multiple LEAFN
leaf blocks.

Hence restore the old behaviour where we skip the DA node tree
rebuild if there is a LEAFN root block found as there is no tree to
rebuild.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>