netfilter: ipv6: fix overlap check for fragments

The type of FRAG6_CB(prev)->offset is int, skb->len is *unsigned* int,
and offset is int.

Without this patch, type conversion occurred to this expression, when
(FRAG6_CB(prev)->offset + prev->len) is less than offset.

Signed-off-by: Shan Wei <shanwei@cn.fujitsu.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>