kernel - opencrypto - optimize chained synchronous callbacks for soft crypto

kernel - opencrypto - optimize chained synchronous callbacks for soft crypto

* When a crypto operation is dispatched to a crypto thread and makes a
  synchronous callback from that thread, and the callback function
  dispatches a followup crypto as part of a chain, we ignore F_BATCH
  and run the followup crypto directly.

  Since we are already in a crypto thread there's no point dispatching
  the followup crypto to another crypto thread.

  Increases dm_crypt's essiv performance w/software crypto by 7%.

* Warning: The chain length for crypto operations with synchronous
  callbacks should be limited to 2 or 3 to avoid blowing up the kernel's
  thread stack.