HAMMER VFS - Fix deadlock which can occur under severe filesystem pressure

HAMMER VFS - Fix deadlock which can occur under severe filesystem pressure

* Inode reflushes (a fsync occuring while the inode is still queued for
  a prior fsync) were not ensuring that the inode got pushed to the backend
  flusher.

  This could lead to deadlocks when the process trying to issue the flush
  is the syncer itself.

* The problem typically occured under filesystem loads where a large number
  of inodes (aka due to a bulk build) are being flushed at once, and the
  flush is unable to finish running before the next syncer cycle comes
  around.