core: deduplicate identical dm-verity ExtensionImages=

It turns out OverlayFS doesn't handle gracefully when the same source is
specified multiple times in lowerdir= and it fails with ELOOP:

Failed to mount overlay (type overlay) on /run/systemd/mount-rootfs/opt (MS_RDONLY "lowerdir=/run/systemd/unit-extensions/1/opt:/run/systemd/unit-extensions/0/opt:/run/systemd/mount-rootfs/opt"): Too many levels of symbolic links

This happens even if we mount each image in a different internal mount
path, as OverlayFS will resolve it and look for the backing device, which
will be the same device mapper entity, and return a hard error.
This error does not appear if dm-verity is not used, so it is very
confusing for users, and unnecessary.

When mounting ExtensionImages, check if an image is dm-veritied,
and drop duplicates if the root hashes match, to avoid this user-unfriendly
hard error.

src/shared/mount-util.c

@@ -941,6 +941,7 @@ static int mount_in_namespace_legacy(
                                 /* required_host_os_release_sysext_level= */ NULL,
                                 /* required_host_os_release_confext_level= */ NULL,
                                 /* required_sysext_scope= */ NULL,
+                                /* verity= */ NULL,
                                 /* ret_image= */ NULL);
         else
                 r = mount_follow_verbose(LOG_DEBUG, FORMAT_PROC_FD_PATH(chased_src_fd), mount_tmp, NULL, MS_BIND, NULL);
@@ -1166,6 +1167,7 @@ static int mount_in_namespace(
                                 /* required_host_os_release_sysext_level= */ NULL,
                                 /* required_host_os_release_confext_level= */ NULL,
                                 /* required_sysext_scope= */ NULL,
+                                /* verity= */ NULL,
                                 &img);
                 if (r < 0)
                         return log_debug_errno(