mount: convert uid and gid properties according with userns

uid and gid are shown in the init userns. We are going to restore mounts
in a target userns, so we need to set these options in the target
userns.

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>