net: use proper lockdep annotation in __sk_dst_set()

__sk_dst_set() must be called while we own the socket.

We can get proper lockdep coverage using lockdep_sock_is_held()
and rcu_dereference_protected()

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