udev: re-introduce symlinks for loopback block device

But the directories are changed from /dev/loop/by-ref/ -> /dev/disk/by-loop-ref/
and /dev/loop/by-inode/ -> /dev/disk/by-loop-inode/.
As /dev/loop/ is used by losetup command for other purpose.
See issue #28475.

This effectively reverts commits 9915cc6086,
5022fab15f, and
c0d998248e.

src/shared/loop-util.c

@@ -1156,7 +1156,11 @@ int loop_device_set_filename(LoopDevice *d, const char *name) {
         /* Sets the .lo_file_name of the loopback device. This is supposed to contain the path to the file
          * backing the block device, but is actually just a free-form string you can pass to the kernel. Most
          * tools that actually care for the backing file path use the sysfs attribute file loop/backing_file
-         * which is a kernel generated string, subject to file system namespaces and such. */
+         * which is a kernel generated string, subject to file system namespaces and such.
+         *
+         * .lo_file_name is useful since userspace can select it freely when creating a loopback block
+         * device, and we can use it for /dev/disk/by-loop-ref/ symlinks, and similar, so that apps can
+         * recognize their own loopback files. */
 
         if (name && strlen(name) >= sizeof(info.lo_file_name))
                 return -ENOBUFS;