discover-image: restore compatibility with C9S and overlayfs directories (#40616)

5817c73391b5f3599c50df2c0873b26ea426f848 broke compatibility with CentOS
9 and overlayfs directories, the following fails with -EOPTNOTSUPP:

mount -t overlay overlay -o lowerdir=/tmp/app1:/tmp/rootdir /tmp/overlay
portablectl attach --copy=symlink --now --runtime /tmp/overlay app1

name_to_handle_at() fails both with and without AT_HANDLE_MNT_ID_UNIQUE.

Restore the fallback to path_get_mnt_id_at() that was removed. Fixes
TEST-29-PORTABLE.directory

Follow-up for 5817c73391b5f3599c50df2c0873b26ea426f848

mkosi/mkosi.conf.d/debian-ubuntu/mkosi.conf

@@ -48,7 +48,6 @@ Packages=
         hostname
         iproute2
         iputils-ping
-        isc-dhcp-server
         knot
         libcap-ng-utils
         libdw-dev

mkosi/mkosi.conf.d/debian-ubuntu/mkosi.conf.d/isc-dhcp.conf

@@ -0,0 +1,16 @@
+# SPDX-License-Identifier: LGPL-2.1-or-later
+
+[TriggerMatch]
+Distribution=debian
+Release=bullseye
+Release=bookworm
+Release=trixie
+
+[TriggerMatch]
+Distribution=ubuntu
+Release=jammy
+Release=noble
+Release=questing
+
+[Content]
+Packages=isc-dhcp-server

src/shared/discover-image.c

@@ -455,10 +455,26 @@ static int image_make(
         uint64_t on_mount_id;
         int _mnt_id;
 
+        /* The fallback is required for CentOS 9 compatibility when working on a directory located on an
+         * overlayfs. */
         r = name_to_handle_at_try_fid(fd, /* path= */ NULL, &fh, &_mnt_id, &on_mount_id, AT_EMPTY_PATH);
+        if (r < 0) {
+                if (is_name_to_handle_at_fatal_error(r))
+                        return r;
+
+                r = path_get_unique_mnt_id_at(fd, /* path= */ NULL, &on_mount_id);
+                if (r < 0) {
+                        if (!ERRNO_IS_NEG_NOT_SUPPORTED(r) && r != -EUNATCH)
+                                return r;
+
+                        int on_mount_id_fallback = -1;
+                        r = path_get_mnt_id_at(fd, /* path= */ NULL, &on_mount_id_fallback);
                         if (r < 0)
                                 return r;
-        if (r == 0)
+
+                        on_mount_id = on_mount_id_fallback;
+                }
+        } else if (r == 0)
                 on_mount_id = _mnt_id;
 
         if (S_ISDIR(st->st_mode)) {