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>