netfilter: nft_set_pipapo: use GFP_KERNEL for insertions

An earlier attempt changed this to GFP_KERNEL, but the get helper is
also called for get requests from userspace, which uses rcu.

Let the caller pass in the kmalloc flags to allow insertions
to schedule if needed.

Suggested-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Florian Westphal <fw@strlen.de>