random: speed up the initialization of module

During the module initialization phase, entropy will be added
to entropy pool for every interrupt, the change should speed up
initialization of the random module.

Before optimization:
[   22.180236] random: crng init done

After optimization:
[    1.474832] random: crng init done
Signed-off-by: NXingjun Liu <xingjun.lxj@alibaba-inc.com>
Reviewed-by: NLiu Jiang <gerry@linux.alibaba.com>
Reviewed-by: NCaspar Zhang <caspar@linux.alibaba.com>
Reviewed-by: Jia Zhang <zhang.jia@linux.alibaba.com>
Reviewed-by: NYang Shi <yang.shi@linux.alibaba.com>
Reviewed-by: NLiu Bo <bo.liu@linux.alibaba.com>

drivers/char/random.c

@@ -1306,7 +1306,8 @@ void add_interrupt_randomness(int irq, int irq_flags)
 	}
 
 	if ((fast_pool->count < 64) &&
-	    !time_after(now, fast_pool->last + HZ))
+	    !time_after(now, fast_pool->last + HZ) &&
+	    crng_ready())
 		return;
 
 	if (!seed_inited) {