[Bug 291420] [epair] Bad UDP packet checksum with epair(4) and txcsum enabled

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=291420

--- Comment #7 from Dmitry Lukhtionov <dmitryluhtionov@gmail.com> ---
I searched my update history.

Previous version on host and jail machine - 14.3-STABLE from 2025-08-22

I have one more jail with epair on this host machine

```
bridge1: flags=1008843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST,LOWER_UP> metric
0 mtu 1500
        options=0
        ether 58:9c:fc:10:ff:ca
        id 00:00:00:00:00:00 priority 32768 hellotime 2 fwddelay 15
        maxage 20 holdcnt 6 proto rstp maxaddr 2000 timeout 1200
        root id 00:00:00:00:00:00 priority 32768 ifcost 0 port 0
        member: epair1a flags=143<LEARNING,DISCOVER,AUTOEDGE,AUTOPTP>
                ifmaxaddr 0 port 19 priority 128 path cost 2000
        member: epair0a flags=143<LEARNING,DISCOVER,AUTOEDGE,AUTOPTP>
                ifmaxaddr 0 port 17 priority 128 path cost 2000
        member: bge1 flags=143<LEARNING,DISCOVER,AUTOEDGE,AUTOPTP>
                ifmaxaddr 0 port 2 priority 128 path cost 55
        groups: bridge

epair0a: flags=1008943<UP,BROADCAST,RUNNING,PROMISC,SIMPLEX,MULTICAST,LOWER_UP>
metric 0 mtu 1500
        options=200009<RXCSUM,CXCSUM,VLAN_MTU,RXCSUM_IPV6>
        ether 02:15:ec:28:01:0a
        groups: epair
        media: Ethernet 10Gbase-T (10Gbase-T <full-duplex>)
        status: active
```
Iside jail
```
epair0b: flags=1008843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST,LOWER_UP> metric
0 mtu 1500
        options=20000b<RXCSUM,TXCSUM,VLAN_MTU,RXCSUM_IPV6>
        ether 02:15:ec:28:01:0b
        inet 193.34.20.6 netmask 0xfffffff0 broadcast 193.34.20.15
        groups: epair
        media: Ethernet 10Gbase-T (10Gbase-T <full-duplex>)
        status: active
        nd6 options=29<PERFORMNUD,IFDISABLED,AUTO_LINKLOCAL>

root@unbound:/ # drill i.ua @193.34.20.6
;; ->>HEADER<<- opcode: QUERY, rcode: NOERROR, id: 56983
;; flags: qr rd ra ; QUERY: 1, ANSWER: 2, AUTHORITY: 0, ADDITIONAL: 0 
;; QUESTION SECTION:
;; i.ua.        IN      A

;; ANSWER SECTION:
i.ua.   149     IN      A       104.18.3.81
i.ua.   149     IN      A       104.18.2.81

;; AUTHORITY SECTION:

;; ADDITIONAL SECTION:

;; Query time: 0 msec
;; SERVER: 193.34.20.6
;; WHEN: Mon Dec  8 13:03:44 2025
;; MSG SIZE  rcvd: 54
```

BUT
'''
root@unbound:/ # ping 193.34.20.5
PING 193.34.20.5 (193.34.20.5): 56 data bytes
^C
--- 193.34.20.5 ping statistics ---
3 packets transmitted, 0 packets received, 100.0% packet loss
root@unbound:/ # ping 193.34.20.4
PING 193.34.20.4 (193.34.20.4): 56 data bytes
64 bytes from 193.34.20.4: icmp_seq=0 ttl=64 time=0.410 ms
64 bytes from 193.34.20.4: icmp_seq=1 ttl=64 time=0.388 ms
^C
--- 193.34.20.4 ping statistics ---
2 packets transmitted, 2 packets received, 0.0% packet loss
round-trip min/avg/max/stddev = 0.388/0.399/0.410/0.011 ms
root@unbound:/ # ping 8.8.8.8
PING 8.8.8.8 (8.8.8.8): 56 data bytes
64 bytes from 8.8.8.8: icmp_seq=0 ttl=120 time=24.884 ms
64 bytes from 8.8.8.8: icmp_seq=1 ttl=120 time=24.976 ms
^C
--- 8.8.8.8 ping statistics ---
2 packets transmitted, 2 packets received, 0.0% packet loss
round-trip min/avg/max/stddev = 24.884/24.930/24.976/0.046 ms
root@unbound:/ # arp -an
? (193.34.20.5) at c4:34:6b:b8:e4:39 on epair0b expires in 1192 seconds
[ethernet]
? (193.34.20.4) at 94:57:a5:55:a2:21 on epair0b expires in 996 seconds
[ethernet]
? (193.34.20.8) at 02:ef:5d:1e:0d:0b on epair0b expires in 793 seconds
[ethernet]
'''

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