migration/multifd: Forbid spurious wakeups

Now multifd's logic is designed to have no spurious wakeup.  I still
remember a talk to Juan and he seems to agree we should drop it now, and if
my memory was right it was there because multifd used to hit that when
still debugging.

Let's drop it and see what can explode; as long as it's not reaching
soft-freeze.

Reviewed-by: Fabiano Rosas <farosas@suse.de>
Link: https://lore.kernel.org/r/20240202102857.110210-15-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>

migration/multifd.c

@@ -756,7 +756,9 @@ static void *multifd_send_thread(void *opaque)
             p->next_packet_size = 0;
             qatomic_set(&p->pending_job, false);
             qemu_mutex_unlock(&p->mutex);
-        } else if (qatomic_read(&p->pending_sync)) {
+        } else {
+            /* If not a normal job, must be a sync request */
+            assert(qatomic_read(&p->pending_sync));
             p->flags = MULTIFD_FLAG_SYNC;
             multifd_send_fill_packet(p);
             ret = qio_channel_write_all(p->c, (void *)p->packet,
@@ -771,9 +773,6 @@ static void *multifd_send_thread(void *opaque)
             qatomic_set(&p->pending_sync, false);
             qemu_mutex_unlock(&p->mutex);
             qemu_sem_post(&p->sem_sync);
-        } else {
-            qemu_mutex_unlock(&p->mutex);
-            /* sometimes there are spurious wakeups */
         }
     }