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

[ Upstream commit 74bc084327c643499474ba75df485607da37dd6e ]

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

Fixes: e292f05e ("tcp: annotate sk->sk_sndbuf 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>