Perform a lockless read to test whether an entropy havesting fifo is

full, avoiding the cost of mutex operations if it is.  We re-test
once the mutex is acquired to make sure it's still true before doing
the -modify-write part of the read-modify-write.  Note that due to
the maximum fifo depth being pretty deep, this is unlikely to improve
harvesting performance yet.

Approved by:	markm

sys/dev/random/randomdev_soft.c

@@ -296,6 +296,10 @@ random_harvest_internal(u_int64_t somecounter, const void *entropy,
 {
 	struct harvest *event;
 
+	/* Lockless read to avoid lock operations if fifo is full. */
+	if (harvestfifo[origin].count >= RANDOM_FIFO_MAX)
+		return;
+
 	/* Lock the particular fifo */
 	mtx_lock_spin(&harvestfifo[origin].lock);