ZFS panic with r255937

I get the following reproducible panic when doing a zfs send -R | zfs recv
of a well churned file system (snapshots of the ports tree before and
after libiconv update) running a recent current:

panic: solaris assert: dn->dn_maxblkid == 0 &&
(BP_IS_HOLE(&dn->dn_phys->dn_blkptr[0]) || dnode_block_freed(dn, 0)),
file:
/usr/src/sys/modules/zfs/../../cddl/contrib/opensolaris/uts/common/fs/zfs/dnode.c,
line: 598

coredump available.

# uname -a
FreeBSD freebsd10 10.0-ALPHA4 FreeBSD 10.0-ALPHA4 #0 r255937: Sun Sep 29
09:45:21 EDT 2013     kwhite_at_freebsd10:/usr/obj/usr/src/sys/GENERIC  amd64


# zfs list -t snapshot -r tank/ports
NAME                  USED  AVAIL  REFER  MOUNTPOINT
tank/ports_at_20130915  72.5M      -  3.64G  -
tank/ports_at_20130917      0      -  3.65G  -
tank/ports_at_20130918      0      -  3.65G  -
tank/ports_at_20130921  88.6M      -  3.66G  -
tank/ports_at_20130928   352K      -  3.66G  -

# zpool list -v
NAME             SIZE  ALLOC   FREE    CAP  DEDUP  HEALTH  ALTROOT
m_tank          1.81T   831G  1.00T    44%  1.00x  ONLINE  -
  ada4.nop.eli  1.81T   831G  1.00T         -
tank             928G   831G  96.9G    89%  1.00x  ONLINE  -
  ada3.nop.eli   928G   831G  96.9G         -

# zfs send -vR tank/ports_at_20130928 | zfs recv -vF m_tank/xports
... panic eventually ...

# cd /boot/kernel; kgdb kernel /var/crash/vmcore.last
...
(kgdb) up
#5  0xffffffff81d09735 in dnode_reallocate (dn=0xfffff8006dde3000,
ot=DMU_OT_PLAIN_FILE_CONTENTS, blocksize=1024, bonustype=DMU_OT_SA,
bonuslen=168, tx=0xfffff8006d7a2600)
    at
/usr/src/sys/modules/zfs/../../cddl/contrib/opensolaris/uts/common/fs/zfs/dnode.c:596

596                     ASSERT(dn->dn_maxblkid == 0 &&

(kgdb) p dn->dn_maxblkid
$1 = 2

So, no question about why the ASSERT failed.

Sorry, debugging this is *way* beyond me.  Any hints, patches to try?

...keith