Revise dirty/valid rules for Index blocks.

Revise dirty/valid rules for Index blocks.

An Index block is 'Valid' if it contains any index.
So an InoIdx block for a depth=0 inode is never Valid
(as there is data rather than indexes there).

When it comes time to cluster_allocate an Index block,
if it is not Valid, we simple allocated_block it to 0.

An index block is Dirty if there are any children that need incorporation
as well was when incorporate has happened but has not yet been
written.
So an Index block can be Dirty but not Valid (unlike data blocks).