selftests/bpf: Fix msg_verify_data in test_sockmap

selftests/bpf: Fix msg_verify_data in test_sockmap

Function msg_verify_data should have context of bytes_cnt and k instead of
assuming they are zero. Otherwise, test_sockmap with data integrity test
will report some errors. I also fix the logic related to size and index j

1/ 6  sockmap::txmsg test passthrough:FAIL
2/ 6  sockmap::txmsg test redirect:FAIL
7/12  sockmap::txmsg test apply:FAIL
10/11  sockmap::txmsg test push_data:FAIL
11/17  sockmap::txmsg test pull-data:FAIL
12/ 9  sockmap::txmsg test pop-data:FAIL
13/ 1  sockmap::txmsg test push/pop data:FAIL
...
Pass: 24 Fail: 52

After applying this patch, some of the errors are solved, but for push,
pull and pop, we may need more fixes to msg_verify_data, added a TODO

10/11  sockmap::txmsg test push_data:FAIL
11/17  sockmap::txmsg test pull-data:FAIL
12/ 9  sockmap::txmsg test pop-data:FAIL
...
Pass: 37 Fail: 15

Besides, added a custom errno EDATAINTEGRITY for msg_verify_data, we
shall not ignore the error in txmsg_cork case.

Fixes: 753fb2ee0934 ("bpf: sockmap, add msg_peek tests to test_sockmap")
Fixes: 16edddfe3c5d ("selftests/bpf: test_sockmap, check test failure")
Acked-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Zijian Zhang <zijianzhang@bytedance.com>
Link: https://lore.kernel.org/r/20241012203731.1248619-2-zijianzhang@bytedance.com
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>