Combine cleaning and orphan list_heads.

Combine cleaning and orphan list_heads.

A datablock is very rarely both an orphan and requiring cleaning, so
having two list_heads is a waste.

If is an orphan it will have full parent linkage and addresses already
so it will be handled promptly and removed from the cleaning list.

So arrange that if a block wants to be both, it is preferentially on
the cleaning list, and when removed from the cleaning list is gets
added back to the pending_orphan list in case it needs processing.

Note that only directory and inode blocks can ever be orphans so some
optimisation of spinlocks is possible.

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