unix: handle sockets with USK_CALLBACK as external sockets

The USK_CALLBACK flag means that a socket is externel and will be
restored by a plugin. open_unixsk_standalone should not be called to
these sockets.

$ make -C test/others/unix-callback/ run
...
(00.109338)   7471: sk unix: Opening standalone socket (id 0xd ino 0 peer 0x63b)
(00.109376)   7471: Error (criu/sk-unix.c:1128): sk unix: BUG at criu/sk-unix.c:1128

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