kernel - allow PG_NOTMETA to be set on regular files too

kernel - allow PG_NOTMETA to be set on regular files too

* Allow this flag to be set for VM pages associated with regular files
  too, the flag prevents the related VM page from being swapcache'd.

  The flag is set by HAMMER on normal file buffer cache buffers when
  double buffering is enabled to prevent swapcache from caching the
  data twice.

* This also fixes an issue when a large number of files exceeding the
  maxvnode limit are recycled, and double buffering is enabled along
  with vm.swapcache.data_enable.  We do not want swapcache to try to
  cache the pages via the vnode, instead we'd rather it cache them
  via the block device (whos vnode doesn't get recycled).