mountpoints: create parent dirs with correct owner

mountpoints: create parent dirs with correct owner

otherwise unprivileged containers might end up with directories that
they cannot modify since they are owned by the user root in the host
namespace, instead of root inside the container.

note: the problematic behaviour is only exhibited when an intermediate
directory needs to be created, e.g. a mountpoint /test/mp gets mounted,
and /test does not yet exist.

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>