HAMMER 61E/Many: Stabilization, Performance

HAMMER 61E/Many: Stabilization, Performance

* PERFORMANCE: hammer_sync_inode() was generating a new transaction id
  for each inode, causing hammer_btree_do_propagation() to have to
  modify B-Tree nodes all the way to root on a per-file basis when
  syncing a rm -rf.

  Change the code to use the flusher's transaction id so all inodes
  bundled into the same flush group use the same transaction id.

* BUG FIX:  The reblocker was able to blow out the buffer cache with
  dirty data buffers.  Even though HAMMER allows these buffers to be
  flushed to the disk at any time by the kernel, calls to bwillwrite()
  are still needed to prevent a buffer cache deadlock.