hammer2 - Fix deadlocks, bad assertion, improve flushing.

hammer2 - Fix deadlocks, bad assertion, improve flushing.

* Fix a deadlock in checkdirempty().  We must release the lock on oparent
  before following a hardlink.  If after re-locking chain->parent != oparent,
  return EAGAIN to the caller.

* When doing a full filesystem flush, pre-flush the vnodes with a normal
  transaction to try to soak-up all the compression time and avoid stalling
  user process writes for too long once we get inside the formal flush.

* Fix a flush bug.  Flushing a deleted chain is allowed if it is an inode.