selftests: net: ip_defrag: modprobe missing nf_defrag_ipv6 support

selftests: net: ip_defrag: modprobe missing nf_defrag_ipv6 support

Fix ip_defrag.sh when CONFIG_NF_DEFRAG_IPV6=m:

$ sudo ./ip_defrag.sh
+ set -e
+ mktemp -u XXXXXX
+ readonly NETNS=ns-rGlXcw
+ trap cleanup EXIT
+ setup
+ ip netns add ns-rGlXcw
+ ip -netns ns-rGlXcw link set lo up
+ ip netns exec ns-rGlXcw sysctl -w net.ipv4.ipfrag_high_thresh=9000000
+ ip netns exec ns-rGlXcw sysctl -w net.ipv4.ipfrag_low_thresh=7000000
+ ip netns exec ns-rGlXcw sysctl -w net.ipv4.ipfrag_time=1
+ ip netns exec ns-rGlXcw sysctl -w net.ipv6.ip6frag_high_thresh=9000000
+ ip netns exec ns-rGlXcw sysctl -w net.ipv6.ip6frag_low_thresh=7000000
+ ip netns exec ns-rGlXcw sysctl -w net.ipv6.ip6frag_time=1
+ ip netns exec ns-rGlXcw sysctl -w net.netfilter.nf_conntrack_frag6_high_thresh=9000000
+ cleanup
+ ip netns del ns-rGlXcw

$ ls -la /proc/sys/net/netfilter/nf_conntrack_frag6_high_thresh
ls: cannot access '/proc/sys/net/netfilter/nf_conntrack_frag6_high_thresh': No such file or directory

$ sudo modprobe nf_defrag_ipv6
$ ls -la /proc/sys/net/netfilter/nf_conntrack_frag6_high_thresh
-rw-r--r-- 1 root root 0 Jul 14 12:34 /proc/sys/net/netfilter/nf_conntrack_frag6_high_thresh

Signed-off-by: Paolo Pisati <paolo.pisati@canonical.com>
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>