Use a PrefixFilter() to get only the mounts we're interested in,
which removes the need to manually filter mounts from the mountinfo
results.
Additional optimizations can be made, as:
> ... there's a little known fact that `umount(MNT_DETACH)` is actually
> recursive in Linux, IOW this function can be replaced with
> `unix.Umount(target, unix.MNT_DETACH)` (or `mount.UnmountAll(target, unix.MNT_DETACH)`
> (provided that target itself is a mount point).
e8fb2c392f (r535450446)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
2374178c9b