netfilter: nf_tables: validate .maxattr at expression registration

struct nft_expr_info allows to store up to NFT_EXPR_MAXATTR (16)
attributes when parsing netlink attributes.

Rise a warning in case there is ever a nft expression whose .maxattr
goes beyond this number of expressions, in such case, struct nft_expr_info
needs to be updated.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>