Fix the use after nil check on args.MountNamespaceVFS2

The args.MountNamespaceVFS2 is used again after the nil check,
instead, mntnsVFS2 which holds the expected reference should be
used. This patch fixes this issue.

Fixes: #3855

Signed-off-by: Tiwei Bie <tiwei.btw@antgroup.com>

pkg/sentry/kernel/kernel.go

@@ -888,17 +888,18 @@ func (k *Kernel) CreateProcess(args CreateProcessArgs) (*ThreadGroup, ThreadID,
 		opener    fsbridge.Lookup
 		fsContext *FSContext
 		mntns     *fs.MountNamespace
+		mntnsVFS2 *vfs.MountNamespace
 	)
 
 	if VFS2Enabled {
-		mntnsVFS2 := args.MountNamespaceVFS2
+		mntnsVFS2 = args.MountNamespaceVFS2
 		if mntnsVFS2 == nil {
 			// MountNamespaceVFS2 adds a reference to the namespace, which is
 			// transferred to the new process.
 			mntnsVFS2 = k.globalInit.Leader().MountNamespaceVFS2()
 		}
 		// Get the root directory from the MountNamespace.
-		root := args.MountNamespaceVFS2.Root()
+		root := mntnsVFS2.Root()
 		// The call to newFSContext below will take a reference on root, so we
 		// don't need to hold this one.
 		defer root.DecRef(ctx)
@@ -1008,7 +1009,7 @@ func (k *Kernel) CreateProcess(args CreateProcessArgs) (*ThreadGroup, ThreadID,
 		UTSNamespace:            args.UTSNamespace,
 		IPCNamespace:            args.IPCNamespace,
 		AbstractSocketNamespace: args.AbstractSocketNamespace,
-		MountNamespaceVFS2:      args.MountNamespaceVFS2,
+		MountNamespaceVFS2:      mntnsVFS2,
 		ContainerID:             args.ContainerID,
 	}
 	t, err := k.tasks.NewTask(config)