drbd: fix decoding of bitmap vli rle for device sizes > 64 TB

Symptoms: disconnect after bitmap exchange due to
bitmap overflow (e:49731075554) while decoding bm RLE packet

In the decoding step of the variable length integer run length encoding
there was potentially an uncatched bitshift by wordsize (variable >> 64).

The result of which is "undefined" :(
(only "sometimes" the result is the desired 0)

Fix: don't do any bit shift magic for shift == 64, just assign.
Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/drbd/drbd_receiver.c

@@ -4125,7 +4125,11 @@ recv_bm_rle_bits(struct drbd_conf *mdev,
 				(unsigned int)bs.buf_len);
 			return -EIO;
 		}
-		look_ahead >>= bits;
+		/* if we consumed all 64 bits, assign 0; >> 64 is "undefined"; */
+		if (likely(bits < 64))
+			look_ahead >>= bits;
+		else
+			look_ahead = 0;
 		have -= bits;
 
 		bits = bitstream_get_bits(&bs, &tmp, 64 - have);