panic: mb_dtor_pack: ref_cnt != 1

I got this panic shortly after boot on a freshly-updated amd64
machine:

FreeBSD/amd64 (fbsd-amd64.isc.org) (ttyd0)

login: panic: mb_dtor_pack: ref_cnt != 1
cpuid = 3
KDB: enter: panic
[thread pid 1021 tid 100131 ]
Stopped at      kdb_enter+0x31: leave
db> wh
Tracing pid 1021 tid 100131 td 0xffffff0323816a40
kdb_enter() at kdb_enter+0x31
panic() at panic+0x1e6
mb_dtor_pack() at mb_dtor_pack+0x103
uma_zfree_arg() at uma_zfree_arg+0x34
mb_free_ext() at mb_free_ext+0xe9
soreceive() at soreceive+0xafb
soo_read() at soo_read+0x5e
dofileread() at dofileread+0x9e
kern_readv() at kern_readv+0x4f
read() at read+0x4b
syscall() at syscall+0x350
Xfast_syscall() at Xfast_syscall+0xa8
--- syscall (3, FreeBSD ELF64, read), rip = 0x800b7e23c, rsp = 0x7fffffffe1a8, rbp = 0x400 ---

Kris