ip6_offload: check segs for NULL in ipv6_gso_segment.

segs needs to be checked for being NULL in ipv6_gso_segment() before calling
skb_shinfo(segs), otherwise kernel can run into a NULL-pointer dereference:

[   97.811262] BUG: unable to handle kernel NULL pointer dereference at 00000000000000cc
[   97.819112] IP: [<ffffffff816e52f9>] ipv6_gso_segment+0x119/0x2f0
[   97.825214] PGD 0 [   97.827047]
[   97.828540] Oops: 0000 [#1] SMP
[   97.831678] Modules linked in: vhost_net vhost macvtap macvlan nfsv3 rpcsec_gss_krb5
nfsv4 dns_resolver nfs fscache xt_CHECKSUM iptable_mangle ipt_MASQUERADE nf_nat_masquerade_ipv4
iptable_nat nf_nat_ipv4 nf_nat nf_conntrack_ipv4 nf_defrag_ipv4 xt_conntrack nf_conntrack
ipt_REJECT nf_reject_ipv4 tun ebtable_filter ebtables ip6table_filter ip6_tables iptable_filter
bridge stp llc snd_hda_codec_realtek snd_hda_codec_hdmi snd_hda_codec_generic snd_hda_intel
snd_hda_codec edac_mce_amd snd_hda_core edac_core snd_hwdep kvm_amd snd_seq kvm snd_seq_device
snd_pcm irqbypass snd_time...

net/ipv6/ip6_offload.c

@@ -99,7 +99,7 @@ static struct sk_buff *ipv6_gso_segment(struct sk_buff *skb,
 		segs = ops->callbacks.gso_segment(skb, features);
 	}
 
-	if (IS_ERR(segs))
+	if (IS_ERR_OR_NULL(segs))
 		goto out;
 
 	gso_partial = !!(skb_shinfo(segs)->gso_type & SKB_GSO_PARTIAL);