netfilter: nf_defrag_ipv6: allow nf_conntrack_frag6_high_thresh increases

Currently, net.netfilter.nf_conntrack_frag6_high_thresh can only be lowered.

I found this issue while investigating a probable kernel issue
causing flakes in tools/testing/selftests/net/ip_defrag.sh

In particular, these sysctl changes were ignored:
	ip netns exec "${NETNS}" sysctl -w net.netfilter.nf_conntrack_frag6_high_thresh=9000000 >/dev/null 2>&1
	ip netns exec "${NETNS}" sysctl -w net.netfilter.nf_conntrack_frag6_low_thresh=7000000  >/dev/null 2>&1

This change is inline with commit 83619623 ("net/ipfrag: let ip[6]frag_high_thresh
in ns be higher than in init_net")

Fixes: 8db3d41569bb ("netfilter: nf_defrag_ipv6: use net_generic infra")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>