zfs solaris assert panic in 11.0-ALPHA5 r302256

My package buiding machine just crashed with this panic during a
poudriere run:

panic: solaris assert: (dn->dn_phys->dn_nlevels == 0 && db->db_level == 0) || dn->dn_phys->dn_nlevels > db->db_level || dn->dn_next_nlevels[txgoff] > db->db_level || dn->dn_next_nlevels[(tx->tx_txg-1) & TXG_MASK] > db->db_level || dn->dn_next_nlevels[(tx->tx_txg
cpuid = 2
KDB: stack backtrace:
db_trace_self_wrapper() at db_trace_self_wrapper+0x2b/frame 0xfffffe085a435eb0
vpanic() at vpanic+0x182/frame 0xfffffe085a435f30
panic() at panic+0x43/frame 0xfffffe085a435f90
assfail() at assfail+0x1a/frame 0xfffffe085a435fa0
dbuf_dirty() at dbuf_dirty+0x3e8/frame 0xfffffe085a436060
dbuf_dirty() at dbuf_dirty+0xba4/frame 0xfffffe085a436120
dbuf_dirty() at dbuf_dirty+0xba4/frame 0xfffffe085a4361e0
dmu_write_uio_dnode() at dmu_write_uio_dnode+0x129/frame 0xfffffe085a436270
dmu_write_uio_dbuf() at dmu_write_uio_dbuf+0x42/frame 0xfffffe085a4362a0
zfs_freebsd_write() at zfs_freebsd_write+0x87c/frame 0xfffffe085a4364c0
VOP_WRITE_APV() at VOP_WRITE_APV+0x16f/frame 0xfffffe085a4365d0
vn_write() at vn_write+0x218/frame 0xfffffe085a436650
vn_io_fault1() at vn_io_fault1+0x1d2/frame 0xfffffe085a4367b0
vn_io_fault() at vn_io_fault+0x197/frame 0xfffffe085a436830
dofilewrite() at dofilewrite+0x87/frame 0xfffffe085a436880
kern_writev() at kern_writev+0x68/frame 0xfffffe085a4368d0
sys_write() at sys_write+0x84/frame 0xfffffe085a436920
amd64_syscall() at amd64_syscall+0x2db/frame 0xfffffe085a436a30
Xfast_syscall() at Xfast_syscall+0xfb/frame 0xfffffe085a436a30
--- syscall (4, FreeBSD ELF64, sys_write), rip = 0x800b9ddea, rsp = 0x7fffffffe568, rbp = 0x7fffffffe760 ---
KDB: enter: panic

The pool is a two disk mirror.  I'm currently running a scrub.

RAM is ECC.

FreeBSD zipper.catspoiler.org 11.0-ALPHA5 FreeBSD 11.0-ALPHA5 #14 r302256M: Thu Jun 30 00:05:32 PDT 2016     dl_at_zipper.catspoiler.org:/usr/obj/usr/src/sys/GENERIC  amd64