blk: add an upper sanity check on partition adding

65536 should be ludicrous anyway but without it we overflow the
memory computation doing the allocation and badness occurs.

Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/ioctl.c

@@ -41,7 +41,7 @@ static int blkpg_ioctl(struct block_device *bdev, struct blkpg_ioctl_arg __user
 			    sizeof(long long) > sizeof(long)) {
 				long pstart = start, plength = length;
 				if (pstart != start || plength != length
-				    || pstart < 0 || plength < 0)
+				    || pstart < 0 || plength < 0 || partno > 65535)
 					return -EINVAL;
 			}