zfs panic VERIFY3U

Hi,

After an unplanned power outage where the UPS also failed, when the (15-CURRENT)
system tried to come back up, it panicked and dropped to debugger. It seems
this happens after zfs.ko loads and then imports the pool. The pool is
raidz2 with 7*4Tb disks. The machine was building something at the time of
the outage. There was probably a 'medium' amount of disk I/O but it wasn't
thrashing.

zpool import causes the following output at the console:
https://void.f-m.fm.user.fm/panic/zpool-broken.png

Is there anything I can do to rescue the pool? Not tried importing it
readonly yet. The system is at main-n278643. It's *not* root-on-zfs.
The boot disk is ufs2.
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