net: Move IP_ROUTER_ALERT out of lock_sock(sk)

ip_ra_control() does not need sk_lock. Who are the another
users of ip_ra_chain? ip_mroute_setsockopt() doesn't take
sk_lock, while parallel IP_ROUTER_ALERT syscalls are
synchronized by ip_ra_lock. So, we may move this command
out of sk_lock.

Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: David S. Miller <davem@davemloft.net>