netfilter: nf_tables: underflow in nft_parse_u32_check()

We don't want to allow negatives here.

Fixes: 36b701fa

 ('netfilter: nf_tables: validate maximum value of u32 netlink attributes')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>