netfilter: nft_lookup: use nft_validate_register_store() to validate types

In preparation of validating the length of a register store, use
nft_validate_register_store() in nft_lookup instead of open coding the
validation.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>