r323412: Panic on boot (slab->us_keg == keg)

I've recently tried to upgrade a HEAD VM (running on a Linux host with
QEMU) from r321082 to r323412.

The new kernel panics right after I try to boot into it with:

panic: Assertion slab->us_keg == keg failed at /usr/src/sys/vm/uma_core.c:2285
cpuid = 0
time = 1
KDB: stack backtrace:
db_trace_self_wrapper() at db_trace_self_wrapper+0x2b/frame 0xffffffff81c4d780
vpanic() at vpanic+0x19c/frame 0xffffffff81c4d800
kassert_panic() at kassert_panic+0x126/frame 0xffffffff81c4d870
keg_fetch_slab() at keg_fetch_slab+0x2a9/frame 0xffffffff81c4d8c0
zone_fetch_slab() at zone_fetch_slab+0x51/frame 0xffffffff81c4d8f0
zone_import() at zone_import+0x4f/frame 0xffffffff81c4d960
zone_alloc_item() at zone_alloc_item+0x36/frame 0xffffffff81c4d9a0
uma_zcreate() at uma_zcreate+0x3d3/frame 0xffffffff81c4da40
uma_startup() at uma_startup+0x147/frame 0xffffffff81c4dae0
vm_page_startup() at vm_page_startup+0x34e/frame 0xffffffff81c4db30
vm_mem_init() at vm_mem_init+0x1a/frame 0xffffffff81c4db50
mi_startup() at mi_startup+0x9c/frame 0xffffffff81c4db70
btext() at btext+0x2c
KDB: enter: panic
[ thread 0 pid 0 tid 0 ]