Hmm, not familiar. Backtrace:
db:0:kdb.enter.default> bt
Tracing pid 87885 tid 100340 td 0xfffffe006a4e7ac0
kdb_enter() at kdb_enter+0x32/frame 0xfffffe0063881830
vpanic() at vpanic+0x163/frame 0xfffffe0063881960
panic() at panic+0x43/frame 0xfffffe00638819c0
trap_fatal() at trap_fatal+0x40c/frame 0xfffffe0063881a20
calltrap() at calltrap+0x8/frame 0xfffffe0063881a20
--- trap 0x9, rip = 0xffffffff80ee0935, rsp = 0xfffffe0063881af0, rbp = 0xfffffe0063881ba0 ---
in_joingroup_locked() at in_joingroup_locked+0x335/frame 0xfffffe0063881ba0
inp_setmoptions() at inp_setmoptions+0x11c6/frame 0xfffffe0063881d30
sosetopt() at sosetopt+0xb4/frame 0xfffffe0063881d80
kern_setsockopt() at kern_setsockopt+0xa5/frame 0xfffffe0063881de0
sys_setsockopt() at sys_setsockopt+0x24/frame 0xfffffe0063881e00
amd64_syscall() at amd64_syscall+0x109/frame 0xfffffe0063881f30
fast_syscall_common() at fast_syscall_common+0xf8/frame 0xfffffe0063881f30
--- syscall (105, FreeBSD ELF64, setsockopt), rip = 0x82687dd0a, rsp = 0x8203704f8, rbp = 0x820370550 ---
Panic:
<6>vlan0: changing name to 'ixv0.7'
<6>ixv0: link state changed to DOWN
<6>ixv0: link state changed to UP
arprequest_internal: cannot find matching address
Fatal trap 9: general protection fault while in kernel mode
cpuid = 6; apic id = 06
instruction pointer = 0x20:0xffffffff80ee0935
stack pointer = 0x28:0xfffffe0063881af0
frame pointer = 0x28:0xfffffe0063881ba0
code segment = base 0x0, limit 0xfffff, type 0x1b
= DPL 0, pres 1, long 1, def32 0, gran 1
processor eflags = interrupt enabled, resume, IOPL = 0
current process = 87885 (avahi-daemon)
rdi: fffff80003c2c480 rsi: 0000000000000000 rdx: 00000000000000a0
rcx: 0000000000000020 r8: 0000000000000000 r9: 00000000000000f8
rax: 14d81dff33337b20 rbx: fffffe006a4e7ac0 rbp: fffffe0063881ba0
r10: 0000000000000000 r11: fffffe006a4e7fe0 r12: 0000000000000000
r13: fffffe0063881c04 r14: fffff800797d9800 r15: fffff80003c2c400
trap number = 9
panic: general protection fault
cpuid = 6
time = 1734052404
KDB: enter: panic
It's interesting that it seems to re-create the interface after boot. Did you resave something there?
I'd have to guess it's some hardware off-loading in the ixv driver causing problem there when it tried to jpin the multicast group.
What hardware offloading is shown as capable or enabled by ifconfig -m ixv0 or for ifconfig -m ixv0.7?