multifd: Fix hang if send thread errors during sync

When a send thread encounters an error (as is the case with yank),
it sets multifd_send_state->exiting and the other threads exit too.
This races with multifd_send_sync_main() which now hangs at
qemu_sem_wait(&p->sem_sync) in multifd_send_sync_main() line 647
as it waits for threads that have exited.

Fix this by kicking the semaphores when exiting the send threads.

I encountered this hang when stress testing the colo unit test,
though I was unable to write a migration test to reliably hit this.

Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Lukas Straub <lukasstraub2@web.de>
Link: https://lore.kernel.org/qemu-devel/20260302-colo_unit_test_multifd-v11-18-d653fb3b1d80@web.de
Signed-off-by: Fabiano Rosas <farosas@suse.de>

migration/multifd.c

@@ -772,9 +772,14 @@ out:
         assert(local_err);
         trace_multifd_send_error(p->id);
         multifd_send_error_propagate(local_err);
-        multifd_send_kick_main(p);
     }
 
+    /*
+     * Always kick the main thread: The main thread might wait on this thread
+     * while another thread encounters an error and signals this thread to exit.
+     */
+    multifd_send_kick_main(p);
+
     rcu_unregister_thread();
     trace_multifd_send_thread_end(p->id, p->packets_sent);