Don't create source directory while the daemon is being shutdown, fix #30348

If a container mount the socket the daemon is listening on into
container while the daemon is being shutdown, the socket will
not exist on the host, then daemon will assume it's a directory
and create it on the host, this will cause the daemon can't start
next time.

fix issue https://github.com/moby/moby/issues/30348

To reproduce this issue, you can add following code

```
--- a/daemon/oci_linux.go
+++ b/daemon/oci_linux.go
@@ -8,6 +8,7 @@ import (
        "sort"
        "strconv"
        "strings"
+       "time"

        "github.com/Sirupsen/logrus"
        "github.com/docker/docker/container"
@@ -666,7 +667,8 @@ func (daemon *Daemon) createSpec(c *container.Container) (*libcontainerd.Spec, e
        if err := daemon.setupIpcDirs(c); err != nil {
                return nil, err
        }
-
+       fmt.Printf("===please stop the daemon===\n")
+       time.Sleep(time.Second * 2)
        ms, err := daemon.setupMounts(c)
        if err != nil {
                return nil, err

```

step1 run a container which has `--restart always` and `-v /var/run/docker.sock:/sock`
```
$ docker run -ti --restart always -v /var/run/docker.sock:/sock busybox
/ #

```
step2 exit the the container
```
/ # exit
```
and kill the daemon when you see
```
===please stop the daemon===
```
in the daemon log

The daemon can't restart again and fail with `can't create unix socket /var/run/docker.sock: is a directory`.

Signed-off-by: Lei Jitang <leijitang@huawei.com>

daemon/volumes_unix.go

@@ -6,6 +6,7 @@ package daemon
 
 import (
 	"encoding/json"
+	"fmt"
 	"os"
 	"path/filepath"
 	"sort"
@@ -42,8 +43,19 @@ func (daemon *Daemon) setupMounts(c *container.Container) ([]container.Mount, er
 		if err := daemon.lazyInitializeVolume(c.ID, m); err != nil {
 			return nil, err
 		}
+		// If the daemon is being shutdown, we should not let a container start if it is trying to
+		// mount the socket the daemon is listening on. During daemon shutdown, the socket
+		// (/var/run/docker.sock by default) doesn't exist anymore causing the call to m.Setup to
+		// create at directory instead. This in turn will prevent the daemon to restart.
+		checkfunc := func(m *volume.MountPoint) error {
+			if _, exist := daemon.hosts[m.Source]; exist && daemon.IsShuttingDown() {
+				return fmt.Errorf("Could not mount %q to container while the daemon is shutting down", m.Source)
+			}
+			return nil
+		}
+
 		rootUID, rootGID := daemon.GetRemappedUIDGID()
-		path, err := m.Setup(c.MountLabel, rootUID, rootGID)
+		path, err := m.Setup(c.MountLabel, rootUID, rootGID, checkfunc)
 		if err != nil {
 			return nil, err
 		}