kernel - Do a better job with the filesystem background sync

kernel - Do a better job with the filesystem background sync

* Adjust code for MNT_LAZY, MNT_NOWAIT, and MNT_WAITOK to reflect
  the fact that they are three different flags and not enumeration
  constants.

* HAMMER now sets VMSC_ONEPASS for MNT_LAZY syncs (background filesystem
  sync).  This generally reduces instances where the background sync
  winds up running continuously when heavy filesystem ops saturate
  the disk.  Fewer vnodes dirtied after the sync is initiated will get
  caught up in the sync.