hammer2 - multi-target mount part 3/many (stabilization)

* Fix cluster memory leaks, fix a malloc_type leak.

* Resync cached PFS (pmp) data in the root inode for the PFS on flush.
  (inode allocator).

* Track total DIO memory.

* For now assert on logical mirror mismatches.

* Minor start on infrastructure for mirror synchronization.

* Refactor hammer2_cluster_copy() flags.