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>

net/ipv6/netfilter/nf_conntrack_reasm.c

@@ -286,7 +286,7 @@ found:
 
 	/* Check for overlap with preceding fragment. */
 	if (prev &&
-	    (NFCT_FRAG6_CB(prev)->offset + prev->len) - offset > 0)
+	    (NFCT_FRAG6_CB(prev)->offset + prev->len) > offset)
 		goto discard_fq;
 
 	/* Look for overlap with succeeding segment. */