Otherwise the kernel will umount when it gets around to it, but
that on lxc_destroy we may race with it and fail the rmdir of
the overmounted (BUSY) rootfs.

This makes lxc-test-snapshot pass for me again.
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>