RPI3, panic on warm boot at r334894

[bob prohaska]

For the first time in months my Pi3 got through a -j4 buildworld/installworld
cycle. Make clean was run in /usr/src, but no other cleaning was done and the
-DNO_CLEAN option was used in buildworld/buildkernel. 

However, on reboot using shutdown -r now it panic'd, with (in part)
(da0:umass-sim0:0:0:0): Error 5, Retries exhausted
g_vfs_done():da0a[READ(offset=163840, length=32768)]error = 5
panic: mount: lost mount
cpuid = 3
time = 1528671592
KDB: stack backtrace:
[console output is at http://www.zefox.net/~fbsd/rpi3/crashes/20180610/]

On a whim, I power-cycled the machine, and it came back up just fine.
Sources were updated to 334935 and a new build/install cycle is running,
again using -DNO_CLEAN to speed things along, without any cleaning.

This is the second occurrence in the last couple of days. Last time
the system was rebooted (warm)  to the old kernel, this time a complete
power cycle cleared the error.

Might the -DNO_CLEAN be the culprit? I expected it would cause compile
time problems, if any.

Thanks for reading, and any guidance.

bob prohaska