utils: handle > 2^31 in lxc_unstack_mountpoint()
utils: handle > 2^31 in lxc_unstack_mountpoint()

I mean.. really? But better safe than sorry.

Signed-off-by: Christian Brauner <48455ab3070520a2d174545c7239d6d0fabd9a83@ubuntu.com>

aeris/lxc,ss1h2a3tw/lxc,terceiro/lxc,LynxChaus/lxc,ss1h2a3tw/lxc,hallyn/lxc,aeris/lxc,caglar10ur/lxc,thmo/lxc,thmo/lxc,federicobriata/lxc,terceiro/lxc,hallyn/lxc,caglar10ur/lxc,LynxChaus/lxc,federicobriata/lxc,aeris/lxc,terceiro/lxc,LynxChaus/lxc,ss1h2a3tw/lxc,thmo/lxc,caglar10ur/lxc,federicobriata/lxc,hallyn/lxc