g_mirror_taste: avoid deadlock, always clear tasting flag

[rlibby]

Summary

If g_mirror_taste encountered an error at g_mirror_add_disk, it might
try to g_mirror_destroy the device with the G_MIRROR_DEVICE_FLAG_TASTING
flag still set. This would wait on a worker to complete the destruction
with g_mirror_try_destroy, but that function bails out if the tasting
flag is set, resulting in a deadlock. Clear the tasting flag before
trying to destroy the device.

Sponsored by: Dell EMC Isilon

Test Plan

sysctl debug.fail_point.mnowait="1%return"
kyua test -k /usr/tests/sys/geom/class/mirror/Kyuafile

[markj]

As a precaution I'd suggest running the tests under /usr/tests/sys/geom/class/mirror.

[rlibby]

In D20744#448546, @markj wrote:

As a precaution I'd suggest running the tests under /usr/tests/sys/geom/class/mirror.

Yes, I believe that is what the kyua line above does (and that's how I found this). Or does that run only a subset of tests?

[markj]

In D20744#448548, @rlibby wrote:

In D20744#448546, @markj wrote:

As a precaution I'd suggest running the tests under /usr/tests/sys/geom/class/mirror.

Yes, I believe that is what the kyua line above does (and that's how I found this). Or does that run only a subset of tests?

Oops, I missed that. No, what you did should run all of the tests.