hv_balloon: rate-limit "Unhandled message" warning

For a couple of times I have encountered a situation where

  hv_balloon: Unhandled message: type: 12447

is being flooded over 1 million times per second with various values,
filling the log and consuming cycles, making debugging difficult.

Add rate limiting to the message.

Most other Hyper-V drivers already have similar rate limiting in their
message callbacks.

The cause of the floods in my case was probably fixed by 96d9d1fa5c
("Drivers: hv: balloon: account for vmbus packet header in
max_pkt_size").

Fixes: 9aa8b50b2b ("Drivers: hv: Add Hyper-V balloon driver")
Signed-off-by: Anssi Hannula <anssi.hannula@bitwise.fi>
Reviewed-by: Michael Kelley <mikelley@microsoft.com>
Link: https://lore.kernel.org/r/20220222141400.98160-1-anssi.hannula@bitwise.fi
Signed-off-by: Wei Liu <wei.liu@kernel.org>

drivers/hv/hv_balloon.c

@@ -1563,7 +1563,7 @@ static void balloon_onchannelcallback(void *context)
 			break;
 
 		default:
-			pr_warn("Unhandled message: type: %d\n", dm_hdr->type);
+			pr_warn_ratelimited("Unhandled message: type: %d\n", dm_hdr->type);
 
 		}
 	}