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).