kerndat: check the SIOCUNIXFILE ioctl for unix sockets

This ioctl opens a file to which a socket is bound and
returns a file descriptor. This file descriptor can be used to get
mnt_id and a file path.

Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>