netfilter: nft_set_pipapo: prefer gfp_kernel allocation

No need to use GFP_ATOMIC here.

Fixes: f6c383b8 ("netfilter: nf_tables: adapt set backend to use GC transaction API")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>