From: Oliver Xymoron <oxymoron@waste.org>

This patch adds locking for SMP. Apparently Willy never managed to
revive his laptop with his version so I revived mine.

The batch pool is copied as a block to avoid long lock hold times
while mixing it into the primary pool. 

Two locks are added:

gobal batch_lock
   batch_entropy_store can be called from any context, and typically from
   interrupts -> spin_lock_irqsave

   batch_entropy_process is called called via schedule_delayed_work and
   runs in process context -> spin_lock_irq

entropy_store.lock
   the mixing process is too expensive to be called from an interrupt
   context and the basic worker function extract_entropy can sleep, so
   all this stuff can be under a normal spin_lock