sk-unix: don't add a socket in a second time

A socket can be added in a list two times, if two other sockets
are connected to it.

This patch fixes a problem:
    Error (sk-unix.c:395): BUG at sk-unix.c:395

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>