ipv4: rcu cleanup in ip_ra_control()

Remove one sparse warning :
net/ipv4/ip_sockglue.c:328:22: warning: incorrect type in assignment (different address spaces)
net/ipv4/ip_sockglue.c:328:22:    expected struct ip_ra_chain [noderef] <asn:4>*next
net/ipv4/ip_sockglue.c:328:22:    got struct ip_ra_chain *[assigned] ra

And replace one rcu_assign_ptr() by RCU_INIT_POINTER() where applicable.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>