attach: use lxc_preserve_ns()
attach: use lxc_preserve_ns()

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

hallyn/lxc,caglar10ur/lxc,thmo/lxc,hfuCN/lxc,cjwatson/lxc,thmo/lxc,federicobriata/lxc,federicobriata/lxc,ysbnim/lxc,ss1h2a3tw/lxc,cjwatson/lxc,GreatFruitOmsk/lxc,terceiro/lxc,GreatFruitOmsk/lxc,cjwatson/lxc,ss1h2a3tw/lxc,terceiro/lxc,LynxChaus/lxc,GreatFruitOmsk/lxc,aeris/lxc,ss1h2a3tw/lxc,thmo/lxc,aeris/lxc,hfuCN/lxc,terceiro/lxc,LynxChaus/lxc,caglar10ur/lxc,caglar10ur/lxc,LynxChaus/lxc,hallyn/lxc,aeris/lxc,hfuCN/lxc,ysbnim/lxc,hallyn/lxc,federicobriata/lxc,ysbnim/lxc