char: hpet: Fix out-of-bounds read bug

Currently, there is an out-of-bounds read on array hpetp->hp_dev
in the following for loop:

870         for (i = 0; i < hdp->hd_nirqs; i++)
871                 hpetp->hp_dev[i].hd_hdwirq = hdp->hd_irq[i];

This is due to the recent change from one-element array to
flexible-array member in struct hpets:

104 struct hpets {
	...
113         struct hpet_dev hp_dev[];
114 };

This change affected the total size of the dynamic memory
allocation, decreasing it by one time the size of struct hpet_dev.

Fix this by adjusting the allocation size when calling
struct_size().

Fixes: 987f028 ("char: hpet: Use flexible-array member")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Acked-by: Eric Biggers <ebiggers@kernel.org>
Link: https://lore.kernel.org/r/20200129022613.GA24281@embeddedor.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/char/hpet.c

@@ -855,7 +855,7 @@ int hpet_alloc(struct hpet_data *hdp)
 		return 0;
 	}
 
-	hpetp = kzalloc(struct_size(hpetp, hp_dev, hdp->hd_nirqs - 1),
+	hpetp = kzalloc(struct_size(hpetp, hp_dev, hdp->hd_nirqs),
 			GFP_KERNEL);
 
 	if (!hpetp)