ip netns: Fix rtnl error while print netns list

Observed on the Linux 3.18:

    # ip netns
    RTNETLINK answers: Operation not supported
    net0

CC: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Fixes: d182ee13 ("ipnetns: allow to get and set netns ids")
Signed-off-by: Vadim Kochan <vadim4j@gmail.com>

configure

@@ -201,7 +201,7 @@ check_setns()
 {
     cat >$TMPDIR/setnstest.c <<EOF
 #include <sched.h>
-int main(int argc, char **argv) 
+int main(int argc, char **argv)
 {
 	(void)setns(0,0);
 	return 0;
@@ -218,6 +218,23 @@ EOF
     rm -f $TMPDIR/setnstest.c $TMPDIR/setnstest
 }
 
+check_netnsid()
+{
+    cat >$TMPDIR/netnsid.c <<EOF
+#include <linux/rtnetlink.h>
+int test_def = RTM_GETNSID;
+EOF
+    $CC -c $TMPDIR/netnsid.c >/dev/null 2>&1
+    if [ $? -eq 0 ]
+    then
+	echo "IP_CONFIG_NETNSID:=y" >> Config
+	echo "yes"
+    else
+	echo "no"
+    fi
+    rm -f $TMPDIR/netnsid.c $TMPDIR/netnsid.o
+}
+
 check_ipset()
 {
     cat >$TMPDIR/ipsettest.c <<EOF
@@ -283,6 +300,8 @@ check_ipt_lib_dir
 
 echo -n "libc has setns: "
 check_setns
+echo -n "netns has peer id suport: "
+check_netnsid
 
 echo -n "SELinux support: "
 check_selinux

ip/Makefile

@@ -16,6 +16,10 @@ ifeq ($(IP_CONFIG_SETNS),y)
 	CFLAGS += -DHAVE_SETNS
 endif
 
+ifeq ($(IP_CONFIG_NETNSID),y)
+	CFLAGS += -DHAVE_NETNSID
+endif
+
 ALLOBJ=$(IPOBJ) $(RTMONOBJ)
 SCRIPTS=ifcfg rtpr routel routef
 TARGETS=ip rtmon

ip/ipnetns.c

@@ -34,6 +34,7 @@ static int usage(void)
 	exit(-1);
 }
 
+#ifdef HAVE_NETNSID
 static int get_netnsid_from_name(const char *name)
 {
 	struct {
@@ -78,6 +79,12 @@ static int get_netnsid_from_name(const char *name)
 
 	return -1;
 }
+#else
+static int get_netnsid_from_name(const char *name)
+{
+	return -1;
+}
+#endif /* HAVE_NETNSID */
 
 static int netns_list(int argc, char **argv)
 {