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

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

Fixes: eac66402

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