rtnetlink: rtnl_link_register() sanity test

Before adding a struct rtnl_link_ops into link_ops list, check it doesnt
clash with a prior one.

Based on a previous patch from Alexander Smirnov

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
CC: Alexander Smirnov <alex.bluesman.smirnov@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>