kernel - Add per-mount token to replace mplock.

kernel - Add per-mount token to replace mplock.

* Fix issues with dupfdopen() not being MPSAFE.

* Implement a dummy mount structure for devfs-synthesized vnodes prior
  to the root mount.

* Wrap all VFS_*() calls, including vfs_init() and vfs_uninit(), to
  acquire the per-mount token if not flagged as being MPSAFE.

* Wrap all VOP_*() calls to acquire the per-mount token if not flagged
  as being MPSAFE.

* Move VOP_READ/VOP_WRITE MPSAFE flags to the mount structure.

* Make fifoops MPSAFE (so HAMMER can flag read & write as being
  MPSAFE generally).

* The VFS code currently also acquires the MP lock when not MPSAFE
  (there are things called by VFSes which are not yet MPSAFE), except
  for read() and write().