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