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

[ Upstream commit e6d12bdb ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>