Revert "rcu: init rcu_registry_lock after fork"

This reverts commit 52437223.
The patch forgot about rcu_sync_lock and was committed by mistake.
Reported-by: NLaszlo Ersek <lersek@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

util/rcu.c

@@ -335,11 +335,6 @@ static void rcu_init_unlock(void)
     qemu_mutex_unlock(&rcu_registry_lock);
     qemu_mutex_unlock(&rcu_sync_lock);
 }
-
-static void rcu_init_child(void)
-{
-    qemu_mutex_init(&rcu_registry_lock);
-}
 #endif
 
 void rcu_after_fork(void)
@@ -351,7 +346,7 @@ void rcu_after_fork(void)
 static void __attribute__((__constructor__)) rcu_init(void)
 {
 #ifdef CONFIG_POSIX
-    pthread_atfork(rcu_init_lock, rcu_init_unlock, rcu_init_child);
+    pthread_atfork(rcu_init_lock, rcu_init_unlock, rcu_init_unlock);
 #endif
     rcu_init_complete();
 }