netfilter: nft_exthdr: fix error handling in nft_exthdr_init()

"err" needs to be signed for the error handling to work.

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>