netfilter: nf_tables: stricter validation of element data

Make sure element data type and length do not mismatch the one specified
by the set declaration.

Fixes: 7d740264

 ("netfilter: nf_tables: variable sized set element keys / data")
Reported-by: Hugues ANGUELKOV <hanguelkov@randorisec.fr>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>