OVN: garbled DNS responses when edns is being used

Steps to reproduce:

1. Have an instance in a tenant network served by ML2/OVN backend
2. Use a dns client that sends queries with edns enabled, e.g. dig from bind9-dnsutils, a common debugging tool.
3. Get garbled responses like:

debian@vm1:~$ dig vm1
;; Warning: Message parser reports malformed message packet.

; <<>> DiG 9.18.16-1~deb12u1-Debian <<>> vm1
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 29669
;; flags: qr rd ad; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0
;; WARNING: recursion requested but not available
;; WARNING: Message has 19 extra bytes at end

;; QUESTION SECTION:
;vm1. IN A

;; ANSWER SECTION:
. 0 CLASS1232 OPT 10 8 ngfvumqOLZU=

;; Query time: 0 msec
;; SERVER: 9.9.9.9#53(9.9.9.9) (UDP)
;; WHEN: Sat Aug 05 19:03:45 UTC 2023
;; MSG SIZE rcvd: 63

Expected result:

Get a valid response like when edns is disabled:

debian@vm1:~$ dig +noedns vm1

; <<>> DiG 9.18.16-1~deb12u1-Debian <<>> +noedns vm1
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 31163
;; flags: qr rd ad; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0
;; WARNING: recursion requested but not available

;; QUESTION SECTION:
;vm1. IN A

;; ANSWER SECTION:
vm1. 3600 IN A 10.128.0.77

;; Query time: 0 msec
;; SERVER: 9.9.9.9#53(9.9.9.9) (UDP)
;; WHEN: Sat Aug 05 19:04:10 UTC 2023
;; MSG SIZE rcvd: 40