Comment # 29 on bug 1175850 from Stanislav Brabec

Fabian Vogt, comment 27: ro mount for subvolumes is not supported with the
recent API at all. So there is no backward incompatible change in fstab
introduced by mountfd API. Just new possible mount options rbind,ro=recursive.
With the recent API, it is interpreted as "rbind,ro". That behavior was
introduced in util-linux-2.39. In the older versions, it was interpreted as
"rbind" and "ro=recursive" passed to the kernel mount as a file system mount
option.