Drivers: hv: balloon: Initialize last_post_time on startup

When left uninitialized, this sometimes fails the following check in
post_status():

	if (!time_after(now, (last_post_time + HZ))) {
		return;
        }

This causes unnecessary delays in reporting memory pressure to host after
booting up.
Signed-off-by: NAlex Ng <alexng@messages.microsoft.com>
Signed-off-by: NK. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/hv/hv_balloon.c

@@ -1655,6 +1655,7 @@ static int balloon_probe(struct hv_device *dev,
 	}
 
 	dm_device.state = DM_INITIALIZED;
+	last_post_time = jiffies;
 
 	return 0;