Second-layer panic (observed on the first patched kernel iteration, which only
allocated if_data_pcpu; boot 20:29 UTC — serial console capture from that boot):

Fatal trap 12: page fault while in kernel mode
cpuid = 0; lapic id = 0
fault virtual address	= 0x0
fault code		= supervisor read data, page not present
instruction pointer	= 0x8:0xffffffff807d4b55
stack pointer	        = 0x10:0xfffff8008d1f89b0
frame pointer	        = 0x10:0xfffff8008d1f8a00
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		= Idle
current thread          = pri 12 
kernel: type 12 trap, code=0

CPU0 stopping CPUs: 0x0000003e
 stopped
Stopped at      in6_ifawithifp+0x35:    movq    (%rax),%rbx

(Reachable because with if_data_pcpu allocated, the decapsulated register now
reaches ip6_input(), which calls in6_ifawithifp(deliverifp=multicast_register_if)
at sys/netinet6/ip6_input.c:531-534; if_addrheads[] is also only allocated by
if_attach() (sys/net/if.c:550-553) and is NULL for the fake ifnet.)