check: Don't forget to init mountns

Otherwise fstatat fails as

> > $ ./crtools check
> > Error (sk-unix.c: 283): Can't stat socket 12547 (./var/run/sdp): Bad file descriptor

Reported-by: "Muralidhar, Rajeev D" <rajeev.d.muralidhar@intel.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
CC: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>