Fetch value and validate u32 netlink attribute. This validation is
usually required when the u32 netlink attributes are being stored in a
field whose size is smaller.

This patch revisits 4da449ae ("netfilter: nft_exthdr: Add size check
on u8 nft_exthdr attributes").

Fixes: 96518518 ("netfilter: add nftables")
Suggested-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Laura Garcia Liebana <nevola@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>