[Bug 282095] enic breaks when changing MTU on interfaces with fib other than 0

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=282095

            Bug ID: 282095
           Summary: enic breaks when changing MTU on interfaces with fib
                    other than 0
           Product: Base System
           Version: 14.1-RELEASE
          Hardware: amd64
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: bugs@FreeBSD.org
          Reporter: freebsd-bugzilla@thismonkey.com

I'm running a Cisco UCS C220M4 with an MLOM card (2 x 10Gb).  I wanted to test
the enic driver for throughput, so I plugged a DAC cable between the ports,
installed a fresh version of 14.1-RELEASE and ran the following:

sysctl net.fibs=3
ifconfig enic0 inet 172.12.1.1/24 fib 1
ifconfig enic1 inet 172.12.2.1/24 fib 2
route add 172.12.2.0/24 -iface enic0 -fib 1
route add 172.12.1.0/24 -iface enic1 -fib 2

I could then ping/iper3 between the interfaces no problem (with setfib 1 ...)

As soon as I alter the MTU of either interface to 9000 bytes, I can no longer
ping, all IP breaks.

PS. The reason I wanted to bump the MTU was I could only get ~3.7Gb/s of UDP
across the interfaces:
[  5]   0.00-10.00  sec  4.27 GBytes  3.67 Gbits/sec  0.001 ms  1142296/4280693
(27%)  receiver
You can see 27% from the sender's side was dropped.  Packets were 1460 bytes.

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