solaris assert: avl_is_empty(&dn -> dn_dbufs) panic

[Don Lewis]

I just started getting this panic:

solaris assert: avl_is_empty(&dn -> dn_dbufs), file:
/usr/src/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/dnode_sync.c,
line 495

System info:
FreeBSD zipper.catspoiler.org 11.0-CURRENT FreeBSD 11.0-CURRENT #25 r286923: Wed Aug 19 09:28:53 PDT 2015     dl at zipper.catspoiler.org:/usr/obj/usr/src/sys/GENERIC  amd64

My zfs pool has one mirrored vdev.  Scrub doesn't find any problems.

%zpool status
  pool: zroot
 state: ONLINE
  scan: scrub repaired 0 in 2h58m with 0 errors on Fri Aug 21 00:44:52 2015
config:

	NAME        STATE     READ WRITE CKSUM
	zroot       ONLINE       0     0     0
	  mirror-0  ONLINE       0     0     0
	    ada0p3  ONLINE       0     0     0
	    ada1p3  ONLINE       0     0     0

This panic is reproduceable and happens every time I use poudriere to
build ports using my 9.3-RELEASE amd64 jail and occurs at the end of the
poudriere run when it is unmounting filesystems.

[00:10:43] ====>> Stopping 4 builders
93amd64-default-job-01: removed
93amd64-default-job-01-n: removed
93amd64-default-job-02: removed
93amd64-default-job-02-n: removed
93amd64-default-job-03: removed
93amd64-default-job-03-n: removed
93amd64-default-job-04: removed
93amd64-default-job-04-n: removed
[00:10:46] ====>> Creating pkgng repository
Creating repository in /tmp/packages: 100%
Packing files for repository: 100%
[00:10:55] ====>> Committing packages to repository
[00:10:55] ====>> Removing old packages
[00:10:55] ====>> Built ports: devel/py-pymtbl net/sie-nmsg net/p5-Net-Nmsg net/axa
[93amd64-default] [2015-08-21_00h47m41s] [committing:] Queued: 4  Built: 4  Failed: 0  Skipped: 0  Ignored: 0  Tobuild: 0   Time: 00:10:53
[00:10:55] ====>> Logs: /var/poudriere/data/logs/bulk/93amd64-default/2015-08-21_00h47m41s
[00:10:55] ====>> Cleaning up
93amd64-default: removed
93amd64-default-n: removed
[00:10:55] ====>> Umounting file systems
Write failed: Broken pipe

Prior to that, I ran poudriere a number of times with a 10.2-STABLE
amd64 jail without incident.

I've kicked off a bunch of poudriere runs for other jails and
will check on it in the morning.