ipv4: get rid of ip_ra_lock

After commit 1215e51e

 ("ipv4: fix a deadlock in ip_ra_control")
we always take RTNL lock for ip_ra_control() which is the only place
we update the list ip_ra_chain, so the ip_ra_lock is no longer needed.

As Eric points out, BH does not need to disable either, RCU readers
don't care.

Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>