ipv6: fix incorrent ipv6 ipsec packet fragment

Since commit 299b076(ipv6: Fix IPsec slowpath fragmentation problem)
In func ip6_append_data,after call skb_put(skb, fraglen + dst_exthdrlen)
the skb->len contains dst_exthdrlen,and we don't reduce dst_exthdrlen at last
This will make fraggap>0 in next "while cycle",and cause the size of skb incorrent

Fix this by reserve headroom for dst_exthdrlen.

Signed-off-by: Gao feng <gaofeng@cn.fujitsu.com>
Acked-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/ip6_output.c

@@ -1414,18 +1414,19 @@ int ip6_append_data(struct sock *sk, int getfrag(void *from, char *to,
 			 */
 			skb->ip_summed = csummode;
 			skb->csum = 0;
-			/* reserve for fragmentation */
-			skb_reserve(skb, hh_len+sizeof(struct frag_hdr));
+			/* reserve for fragmentation and ipsec header */
+			skb_reserve(skb, hh_len + sizeof(struct frag_hdr) +
+				    dst_exthdrlen);
 
 			if (sk->sk_type == SOCK_DGRAM)
 				skb_shinfo(skb)->tx_flags = tx_flags;
 
 			/*
 			 *	Find where to start putting bytes
 			 */
-			data = skb_put(skb, fraglen + dst_exthdrlen);
-			skb_set_network_header(skb, exthdrlen + dst_exthdrlen);
-			data += fragheaderlen + dst_exthdrlen;
+			data = skb_put(skb, fraglen);
+			skb_set_network_header(skb, exthdrlen);
+			data += fragheaderlen;
 			skb->transport_header = (skb->network_header +
 						 fragheaderlen);
 			if (fraggap) {