xen/balloon: make the balloon wait interruptible

So it can be killed, or else processes can get hung indefinitely
waiting for balloon pages.
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20200727091342.52325-3-roger.pau@citrix.comSigned-off-by: Juergen Gross <jgross@suse.com>

drivers/xen/balloon.c

@@ -568,11 +568,13 @@ static int add_ballooned_pages(int nr_pages)
 	if (xen_hotplug_unpopulated) {
 		st = reserve_additional_memory();
 		if (st != BP_ECANCELED) {
+			int rc;
+
 			mutex_unlock(&balloon_mutex);
-			wait_event(balloon_wq,
+			rc = wait_event_interruptible(balloon_wq,
 				   !list_empty(&ballooned_pages));
 			mutex_lock(&balloon_mutex);
-			return 0;
+			return rc ? -ENOMEM : 0;
 		}
 	}