hammer2 - Add peer_type field to LNK_CONN and LNK_SPAN

hammer2 - Add peer_type field to LNK_CONN and LNK_SPAN

* Adds a peer_type field allowing connections to identify what they are
  (i.e. a HAMMER2 mount, a CLUSTER controller, a BLOCK device controller).

  Rename the pfs_type field in the volume header, which never made much
  sense, to peer_type, which now does make sense though for the moment
  the only value it can have is HAMER2_PEER_HAMMER2.

* Filter HAMMER2_PEER_HAMMER2 peer types by pfs_clid to reduce unnecessary
  LNK_SPAN traffic being transmitted to a HAMMER2 mount (in the kernel).

* Minor cleanup.