ipv6: ipv6_find_hdr restore prev functionality

The commit 9195bb8 ("ipv6: improve
ipv6_find_hdr() to skip empty routing headers") broke ipv6_find_hdr().

When a target is specified like IPPROTO_ICMPV6 ipv6_find_hdr()
returns -ENOENT when it's found, not the header as expected.

A part of IPVS is broken and possible also nft_exthdr_eval().
When target is -1 which it is most cases, it works.

This patch exits the do while loop if the specific header is found
so the nexthdr could be returned as expected.

Reported-by: Art -kwaak- van Breemen <ard@telegraafnet.nl>
Signed-off-by: Hans Schillstrom <hans@schillstrom.com>
CC:Ansis Atteka <aatteka@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/exthdrs_core.c

@@ -212,7 +212,7 @@ int ipv6_find_hdr(const struct sk_buff *skb, unsigned int *offset,
 		found = (nexthdr == target);
 
 		if ((!ipv6_ext_hdr(nexthdr)) || nexthdr == NEXTHDR_NONE) {
-			if (target < 0)
+			if (target < 0 || found)
 				break;
 			return -ENOENT;
 		}