Fix kerndat_link_nsid() on systems with more than 10 interfaces

On a system with more than 10 network interfaces the link_nsid check
fails:

$ criu check --feature link_nsid
Warn  (criu/cr-check.c:1237): NSID isn't supported

The function kerndat_link_nsid() uses:

 nde.ifindex = 10;

This fails as there is already an interface with ifindex 10.

This patch moves the creation of the socket into the second network
namespace and the feature check succeeds.

Suggested-by: Jiri Benc <jbenc@redhat.com>
Signed-off-by: Adrian Reber <areber@redhat.com>
Signed-off-by: Andrei Vagin <avagin@gmail.com>