If the kernel is misconfigured (say UDP diag module is not set)
we should detect such situation and report an error.

This patch adds a test for error code. Note the kernel requires
a patch as well. Without kernel patch this test do not harm
anyway, thus safe to merge.

#2412
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>