net/dns/resolver: EDNS OPT record off-by-one

I don't know how to get access to a real packet. Basing this commit
entirely off:

       +------------+--------------+------------------------------+
       | Field Name | Field Type   | Description                  |
       +------------+--------------+------------------------------+
       | NAME       | domain name  | MUST be 0 (root domain)      |
       | TYPE       | u_int16_t    | OPT (41)                     |
       | CLASS      | u_int16_t    | requestor's UDP payload size |
       | TTL        | u_int32_t    | extended RCODE and flags     |
       | RDLEN      | u_int16_t    | length of all RDATA          |
       | RDATA      | octet stream | {attribute,value} pairs      |
       +------------+--------------+------------------------------+

From https://datatracker.ietf.org/doc/html/rfc6891#section-6.1.2

Signed-off-by: David Crawshaw <crawshaw@tailscale.com>

net/dns/resolver/forwarder.go

@@ -125,6 +125,7 @@ func clampEDNSSize(packet []byte, maxSize uint16) {
 		return
 	}
 
+	// https://datatracker.ietf.org/doc/html/rfc6891#section-6.1.2
 	opt := packet[len(packet)-optFixedBytes:]
 
 	if opt[0] != 0 {
@@ -141,8 +142,8 @@ func clampEDNSSize(packet []byte, maxSize uint16) {
 		// Be conservative and don't touch unknown versions.
 		return
 	}
-	// Ignore flags in opt[7:9]
-	if binary.BigEndian.Uint16(opt[10:12]) != 0 {
+	// Ignore flags in opt[6:9]
+	if binary.BigEndian.Uint16(opt[9:11]) != 0 {
 		// RDLEN must be 0 (no variable length data). We're at the end of the
 		// packet so this should be 0 anyway)..
 		return