selftests, bpf: Fix test_txmsg_ingress_parser error

After "skmsg: lose offset info in sk_psock_skb_ingress", the test case
with ktls failed. This because ktls parser(tls_read_size) return value
is 285 not 256.

The case like this:

	tls_sk1 --> redir_sk --> tls_sk2

tls_sk1 sent out 512 bytes data, after tls related processing redir_sk
recved 570 btyes data, and redirect 512 (skb_use_parser) bytes data to
tls_sk2; but tls_sk2 needs 285 * 2 bytes data, receive timeout occurred.

Signed-off-by: Liu Jian <liujian56@huawei.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20211029141216.211899-2-liujian56@huawei.com