lxc-start: don't re-try to mount rootfs if we already did so

lxc-start: don't re-try to mount rootfs if we already did so

If we are root using a user namespace and are mounting a blockdev as rootfs,
then we do this before unsharing the userns, because we are not allowed to
do it in a userns.  But after unsharing the userns, we unconditionally
retried mounting the rootfs, resulting in failure.  stop that.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>