HAMMER VFS - Fix degenerate stall condition in flusher during unmount

HAMMER VFS - Fix degenerate stall condition in flusher during unmount

* Fix a case where the flusher can stall during an unmount.

* Rework the flusher sequence numbers to always allocate a sequence number
  when a flush is requested, remove the flusher.act field, and rejigger the
  code a bit.

* This also cleans up an edge case when a full sync is inserted (when taking
  snapshots, filesystem sync, etc), by inserting several sequence numbers to
  completely flush the UNDO/REDO FIFO before moving on to the next active
  flush group.

Reported-by: Sepherosa Ziehau <sepherosa@gmail.com>, Francois Tigeot <ftigeot@wolfpond.org>, numerous others.