bpf: Selftests, print error in test_sockmap error cases

Its helpful to know the error value if an error occurs.

Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Jakub Sitnicki <jakub@cloudflare.com>
Link: https://lore.kernel.org/bpf/158939724566.15176.12079885932643225626.stgit@john-Precision-5820-Tower