Fix bug where podman mount didn't error as rootless

We require that rootless `podman mount` be run inside a shell
spawned by `podman unshare` (which gives us a mount namespace
which actually lets other commands use the mounted filesystem).

The fix is simple - we need to mark the command as requiring the
rootless user namespace not be configured, so we can test for it
later as part of the mount code and error if we needed to make
one.

Fixes #6856

Signed-off-by: Matthew Heon <matthew.heon@pm.me>

cmd/podman/containers/mount.go

@@ -30,13 +30,18 @@ var (
 		Args: func(cmd *cobra.Command, args []string) error {
 			return validate.CheckAllLatestAndCIDFile(cmd, args, true, false)
 		},
+		Annotations: map[string]string{
+			registry.ParentNSRequired: "",
+		},
 	}
 
 	containerMountCommmand = &cobra.Command{
-		Use:   mountCommand.Use,
-		Short: mountCommand.Short,
-		Long:  mountCommand.Long,
-		RunE:  mountCommand.RunE,
+		Use:         mountCommand.Use,
+		Short:       mountCommand.Short,
+		Long:        mountCommand.Long,
+		RunE:        mountCommand.RunE,
+		Args:        mountCommand.Args,
+		Annotations: mountCommand.Annotations,
 	}
 )