random: Use this_cpu_inc_return

__this_cpu_inc can create a single instruction to do the same as
__get_cpu_var()++.

Cc: Richard Kennedy <richard@rsk.demon.co.uk>
Cc: Matt Mackall <mpm@selenic.com>
Acked-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

drivers/char/random.c

@@ -626,7 +626,7 @@ static void add_timer_randomness(struct timer_rand_state *state, unsigned num)
 	preempt_disable();
 	/* if over the trickle threshold, use only 1 in 4096 samples */
 	if (input_pool.entropy_count > trickle_thresh &&
-	    (__get_cpu_var(trickle_count)++ & 0xfff))
+	    ((__this_cpu_inc_return(trickle_count) - 1) & 0xfff))
 		goto out;
 
 	sample.jiffies = jiffies;