tun: Don't dump tun file if we're not taking netns with us

Tun files are attached to tun links, thus we need the latter to
be dumped together with the former.
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

tun.c

@@ -12,6 +12,7 @@
 #include "files-reg.h"
 #include "tun.h"
 #include "net.h"
+#include "namespaces.h"
 
 #include "protobuf/tun.pb-c.h"
 
@@ -268,6 +269,11 @@ static int dump_tunfile(int lfd, u32 id, const struct fd_parms *p)
 	TunfileEntry tfe = TUNFILE_ENTRY__INIT;
 	struct ifreq ifr;
 
+	if (!(current_ns_mask & CLONE_NEWNET)) {
+		pr_err("Net namespace is required to dump tun link\n");
+		return -1;
+	}
+
 	if (dump_one_reg_file(lfd, id, p))
 		return -1;