rtnetlink: small rtnl lock pushdown

rtnetlink: small rtnl lock pushdown

instead of rtnl lock/unload at the top level, push it down
to the called function.

This is just an intermediate step, next commit switches protection
of the rtnl_link ops table to rcu, in which case (for dumps) the
rtnl lock is acquired only by the netlink dumper infrastructure
(current lock/unlock/dump/lock/unlock rtnl sequence becomes
 rcu lock/rcu unlock/dump).

Signed-off-by: Florian Westphal <[email protected]>
Reviewed-by: Hannes Frederic Sowa <[email protected]>
Signed-off-by: David S. Miller <[email protected]>