namespace: fix MAC labels of /dev when PrivateDevices=yes

Without changing the SELinux label for private /dev of a service, it will take
a generic file system label:
system_u:object_r:tmpfs_t:s0

After this change it is the same as without `PrivateDevices=yes`:
system_u:object_r:device_t:s0

This helps writing SELinux policies, as the same rules for `/dev` will apply
despite any `PrivateDevices=yes` setting.

src/basic/label.h

@@ -9,7 +9,10 @@ typedef enum LabelFixFlags {
         LABEL_IGNORE_EROFS  = 1 << 1,
 } LabelFixFlags;
 
-int label_fix(const char *path, LabelFixFlags flags);
+int label_fix_container(const char *path, const char *inside_path, LabelFixFlags flags);
+static inline int label_fix(const char *path, LabelFixFlags flags) {
+        return label_fix_container(path, path, flags);
+}
 
 int mkdir_label(const char *path, mode_t mode);
 int mkdirat_label(int dirfd, const char *path, mode_t mode);