unix: switch into the root mount namespace before cleaning stale sockets

Absolute paths are used to delete old unix sockets. After 3f67731b8e
all processes live in a mount root yard, so if we want to access
absolute paths, we have to change root.

https://travis-ci.org/avagin/criu/builds/197092365


Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>