When skb->len==0, the recv_actor() returns 0 too, but we also use 0
for error conditions. This patch amends this by propagating the errors
to tcp_read_skb() so that we can distinguish skb->len==0 case from
error cases.

Fixes: 04919bed ("tcp: Introduce tcp_read_skb()")
Reported-by: NEric Dumazet <edumazet@google.com>
Cc: John Fastabend <john.fastabend@gmail.com>
Cc: Jakub Sitnicki <jakub@cloudflare.com>
Signed-off-by: NCong Wang <cong.wang@bytedance.com>
Signed-off-by: NJakub Kicinski <kuba@kernel.org>