xfs: fix online repair block reaping

These patches fix a few problems that I noticed in the code that deals
with old btree blocks after a successful repair. First, we clarify how
the reaping function works w.r.t. bitmap lifetimes.  Next we fix a bug
where we could incorrectly invalidate old btree blocks if they were
crosslinked.  Finally, we convert the reap function to use EFIs so that
we can delete blocks without overloading a transaction.