selftests/bpf: test_sockmap, fix data verification

When data verification is enabled, some tests fail because verification is done
incorrectly. Following changes fix it.

- Identify the size of data block to be verified
- Reset verification counter when data block size is reached
- Fixed the value printed in case of verfication failure

Fixes: 16962b24 ("bpf: sockmap, add selftests")
Acked-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Prashant Bhole <bhole_prashant_q7@lab.ntt.co.jp>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>