bpf: bpf_probe_read_kernel_str() has to return amount of data read on success

During recent refactorings, bpf_probe_read_kernel_str() started returning 0 on
success, instead of amount of data successfully read. This majorly breaks
applications relying on bpf_probe_read_kernel_str() and bpf_probe_read_str()
and their results. Fix this by returning actual number of bytes read.

Fixes: 8d92db5c04d1 ("bpf: rework the compat kernel probe handling")
Signed-off-by: Andrii Nakryiko <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>
Reviewed-by: Christoph Hellwig <[e-mail]>
Acked-by: John Fastabend <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

kernel/trace/bpf_trace.c

···
 	if (unlikely(ret < 0))
 		goto fail;
 
-	return 0;
+	return ret;
 fail:
 	memset(dst, 0, size);
 	return ret;