block: protect tracked_requests and flush_queue with reqs_lock

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20170605123908.18777-14-pbonzini@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>

block.c

@@ -320,6 +320,7 @@ BlockDriverState *bdrv_new(void)
         QLIST_INIT(&bs->op_blockers[i]);
     }
     notifier_with_return_list_init(&bs->before_write_notifiers);
+    qemu_co_mutex_init(&bs->reqs_lock);
     bs->refcnt = 1;
     bs->aio_context = qemu_get_aio_context();