kernel - SWAP CACHE part 19/many - distinguish bulk data in HAMMER block dev

kernel - SWAP CACHE part 19/many - distinguish bulk data in HAMMER block dev

* Add buf->flags/B_NOTMETA, vm_page->flags/PG_NOTMETA.  If set the pages
  underlying the buffer will not be considered meta-data from the
  point of view of the swapcache.

* HAMMER must sometimes access bulk data via the block device instead of
  via a file vnode.  For example, the reblocking and mirroring code.
  We do not want this data to be misinterpreted as meta-data when
  the meta-data-only swapcache is turned on, otherwise it will blow
  out the actual meta-data in the swapcache.

  HAMMER_RECTYPE_DATA and HAMMER_RECTYPE_DB are considered normal data.
  All other record types (e.g. direntry, inode, etc) are meta-data.