net: Prepare to dump netdev entry with extentions

Some (most) network devices would like to have NetDeviceEntry with
more fields, than currently present (and enough for lo and veth).
Prepare for that by allowing them to define their own callback that
would fill the resor of the pb entry and call write_netdev_img().
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

include/net.h

@@ -21,4 +21,7 @@ extern void network_unlock(void);
 
 extern struct ns_desc net_ns_desc;
 
+#include "protobuf/netdev.pb-c.h"
+int write_netdev_img(NetDeviceEntry *nde, struct cr_fdset *fds);
+
 #endif /* __CR_NET_H__ */

net.c

@@ -23,8 +23,14 @@ void show_netdevices(int fd)
 	pb_show_plain_pretty(fd, PB_NETDEV, "2:%d");
 }
 
+int write_netdev_img(NetDeviceEntry *nde, struct cr_fdset *fds)
+{
+	return pb_write_one(fdset_fd(fds, CR_FD_NETDEV), nde, PB_NETDEV);
+}
+
 static int dump_one_netdev(int type, struct ifinfomsg *ifi,
-		struct rtattr **tb, struct cr_fdset *fds)
+		struct rtattr **tb, struct cr_fdset *fds,
+		int (*dump)(NetDeviceEntry *, struct cr_fdset *))
 {
 	NetDeviceEntry netdev = NET_DEVICE_ENTRY__INIT;
 
@@ -39,7 +45,10 @@ static int dump_one_netdev(int type, struct ifinfomsg *ifi,
 	netdev.flags = ifi->ifi_flags;
 	netdev.name = RTA_DATA(tb[IFLA_IFNAME]);
 
-	return pb_write_one(fdset_fd(fds, CR_FD_NETDEV), &netdev, PB_NETDEV);
+	if (!dump)
+		dump = write_netdev_img;
+
+	return dump(&netdev, fds);
 }
 
 static char *link_kind(struct ifinfomsg *ifi, struct rtattr **tb)
@@ -78,7 +87,7 @@ static int dump_one_ethernet(struct ifinfomsg *ifi,
 		 * Sigh... we have to assume, that the veth device is a
 		 * connection to the outer world and just dump this end :(
 		 */
-		return dump_one_netdev(ND_TYPE__VETH, ifi, tb, fds);
+		return dump_one_netdev(ND_TYPE__VETH, ifi, tb, fds, NULL);
 unk:
 	pr_err("Unknown eth kind %s link %d\n", kind, ifi->ifi_index);
 	return -1;
@@ -104,7 +113,7 @@ static int dump_one_link(struct nlmsghdr *hdr, void *arg)
 
 	switch (ifi->ifi_type) {
 	case ARPHRD_LOOPBACK:
-		ret = dump_one_netdev(ND_TYPE__LOOPBACK, ifi, tb, fds);
+		ret = dump_one_netdev(ND_TYPE__LOOPBACK, ifi, tb, fds, NULL);
 		break;
 	case ARPHRD_ETHER:
 		ret = dump_one_ethernet(ifi, tb, fds);