selftests/bpf: Fix SENDPAGE data logic in test_sockmap

selftests/bpf: Fix SENDPAGE data logic in test_sockmap

In the SENDPAGE test, "opt->iov_length * cnt" size of data will be sent
cnt times by sendfile.
1. In push/pop tests, they will be invoked cnt times, for the simplicity of
msg_verify_data, change chunk_sz to iov_length
2. Change iov_length in test_send_large from 1024 to 8192. We have pop test
where txmsg_start_pop is 4096. 4096 > 1024, an error will be returned.

Fixes: 328aa08a081b ("bpf: Selftests, break down test_sockmap into subtests")
Signed-off-by: Zijian Zhang <zijianzhang@bytedance.com>
Reviewed-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/r/20241106222520.527076-3-zijianzhang@bytedance.com
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>