qcow2: call CoQueue APIs under CoMutex

Reviewed-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: NFam Zheng <famz@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Message-Id: <20170629132749.997-2-pbonzini@redhat.com>
Signed-off-by: NFam Zheng <famz@redhat.com>

block/qcow2.c

@@ -2025,8 +2025,6 @@ static coroutine_fn int qcow2_co_pwritev(BlockDriverState *bs, uint64_t offset,
     ret = 0;
 
 fail:
-    qemu_co_mutex_unlock(&s->lock);
-
     while (l2meta != NULL) {
         QCowL2Meta *next;
 
@@ -2040,6 +2038,8 @@ fail:
         l2meta = next;
     }
 
+    qemu_co_mutex_unlock(&s->lock);
+
     qemu_iovec_destroy(&hd_qiov);
     qemu_vfree(cluster_data);
     trace_qcow2_writev_done_req(qemu_coroutine_self(), ret);