Delay hashing of index blocks until they are incorporated.

Delay hashing of index blocks until they are incorporated.

We don't need an index block in the hash table until its
address is in the parent, as until then we will never try a lookup.

And it is good to delay it as it is possible for there to be two
blocks with the same address, one that is empty and thus ignored
mostly, and one that has since split of an earlier child.
While this is unlikely, we don't want that split-off block to
appear in the hash table until both have been incorporated.

Signed-off-by: NeilBrown <neilb@suse.de>