xen/balloon: fix balloon kthread freezing

Commit 8480ed9c ("xen/balloon: use a kernel thread instead a
workqueue") switched the Xen balloon driver to use a kernel thread.
Unfortunately the patch omitted to call try_to_freeze() or to use
wait_event_freezable_timeout(), causing a system suspend to fail.

Fixes: 8480ed9c ("xen/balloon: use a kernel thread instead a workqueue")
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Link: https://lore.kernel.org/r/20210920100345.21939-1-jgross@suse.comSigned-off-by: Juergen Gross <jgross@suse.com>

drivers/xen/balloon.c

@@ -522,8 +522,8 @@ static int balloon_thread(void *unused)
 			timeout = 3600 * HZ;
 		credit = current_credit();
 
-		wait_event_interruptible_timeout(balloon_thread_wq,
-				 balloon_thread_cond(state, credit), timeout);
+		wait_event_freezable_timeout(balloon_thread_wq,
+			balloon_thread_cond(state, credit), timeout);
 
 		if (kthread_should_stop())
 			return 0;