map: don't unmarshal unnecessary items on batch lookup

[ti-mo]

It does seem like Map.batchLookup() always tries to unmarshal the whole keyBuf and valueBuf regardless of the Count received from the batch syscall.

ebpf/map.go

Lines 1076 to 1083 in c739d15

	err := sysenc.Unmarshal(keysOut, keyBuf)
	if err != nil {
	return 0, err
	}
	err = sysenc.Unmarshal(valuesOut, valueBuf)
	if err != nil {
	return 0, err
	}

It would be nice if we instead only decoded count keys and values.

See #1078 (comment).

[Hudsoncoutinho]

err := sysenc.Desempacotamento(keysOut, keyBuf, count)
if err != nil {
return 0, err
}
err = sysenc.Desempacotamento(valuesOut, valueBuf, count)
if err != nil {
return 0, err
}

I thought about providing the actual count of items that need to be unpacked, storing it in the variable count, therefore, the Desempacotamento function needs to be able to accept an item count. If this is not supported, one option would be to adjust the function so that it receives this parameter