hammer2 - Cleanup hammer2_cluster API

hammer2 - Cleanup hammer2_cluster API

* Track whether a cluster structure is locked or not.  For the upcoming
  cluster locking code, cluster structures cannot be recursively locked
  (though you can always make a copy and lock that).

* Move the ddflag (direct-data-in-inode) flag into the cluster structure
  And remove it from a number of API calls.

* Add fields and flags in preparation for more formalized locking and
  tracking of clusters.

* Refactor code to remove hammer2_cluster_replace() and
  hammer2_cluster_replace_locked().  This involves changing
  hammer2_hardlink_find() and a few other things.

* Refactor the cluster focus code.  cluster->focus is only set and valid
  when a cluster is locked.  Do not set it as a side effect of other
  operations.

* Refactor chain validation in hammer2_cluster_lookup() and add it to
  hammer2_cluster_next().

* Add the concept of a read-only cluster lock which will allow upcoming
  cluster locking code to reduce the number of chains that actually
  get locked or undergo I/O.

* In strategy file writing code call hammer2_cluster_modify_ip()
  only when the inode itself is being modified by the write() due
  to being in OPFLAG_DIRECTDATA mode (when 512 bytes of data is embedded
  in the inode).