net: add missing READ_ONCE(sk->sk_rcvbuf) annotation

In a prior commit, I forgot to change sk_getsockopt()
when reading sk->sk_rcvbuf locklessly.

Fixes: ebb3b78d ("tcp: annotate sk->sk_rcvbuf lockless reads")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>