This currently gets used in libsysprof to try to cross mount namespaces to somewhere we can access a binary on the same subvolume.
This lets you get the full string that was parsed from the mountinfo rather than having to go through our yet-to-be-implemented specific option API.
And expose it via sysprof_document_process_list_mounts() so that when inspecting processes we can see what binaries were mapped as well as what the filesystem looked like to locate those mapped paths.
