panic: dead bo 0xc47cf854 on rpi2

[bob prohaska]

While running make clean in /usr/ports an rpi2 running 
FreeBSD www.zefox.com 12.0-CURRENT FreeBSD 12.0-CURRENT #8 r325884: Thu Nov 16 01:28:57 PST 2017     bob at www.zefox.com:/usr/obj/usr/src/arm.armv7/sys/RPI2  arm

reported 

panic: dead bo 0xc47cf854

Console, backtrace and logfile are at 
http://www.zefox.net/~fbsd/rpi2/crashes/crash_11_16_17/

I've never before seen a panic during make clean, in ports or src, so it
was a considerable surprise. One other oddity was that an ssh session
running a top window started to exit and close the ssh session without
errors repeatedly after the make clean started, so it's unlikely the
top window shown has much to do with what was running at the time of
the panic.  

Thanks for reading, and any suggestions.

bob prohaska