unhash iblock when it becomes empty.

unhash iblock when it becomes empty.

Our handling of empty iblocks was flawed - when they become empty they
stayed in the hash table and could be found again - bad.

So unhash them.
To keep things tidy, always use hlist_del_init to removing things from
the hash table, and assert that they are present before removing, and
that they aren't before adding.

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