kernel - Rejigger mount code to add vfs_flags in struct vfsops

kernel - Rejigger mount code to add vfs_flags in struct vfsops

* Rejigger the mount code so we can add a vfs_flags field to vfsops,
  which mount_init() has visibility to.

* Allows nullfs to flag that its mounts do not need a syncer thread.
  Previously nullfs would destroy the syncer thread after the
  fact.

* Improves dsynth performance (it does lots of nullfs mounts).