rtnetlink: add guard for RTNL

The new guard/scoped_gard can be useful for the RTNL as well,
so add a guard definition for it. It gets used like

 {
   guard(rtnl)();
   // RTNL held until end of block
 }

or

  scoped_guard(rtnl) {
    // RTNL held in this block
  }

as with any other guard/scoped_guard.
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/rtnetlink.h

@@ -7,6 +7,7 @@
 #include <linux/netdevice.h>
 #include <linux/wait.h>
 #include <linux/refcount.h>
+#include <linux/cleanup.h>
 #include <uapi/linux/rtnetlink.h>
 
 extern int rtnetlink_send(struct sk_buff *skb, struct net *net, u32 pid, u32 group, int echo);
@@ -46,6 +47,8 @@ extern int rtnl_is_locked(void);
 extern int rtnl_lock_killable(void);
 extern bool refcount_dec_and_rtnl_lock(refcount_t *r);
 
+DEFINE_LOCK_GUARD_0(rtnl, rtnl_lock(), rtnl_unlock())
+
 extern wait_queue_head_t netdev_unregistering_wq;
 extern atomic_t dev_unreg_count;
 extern struct rw_semaphore pernet_ops_rwsem;