geli: fix a livelock during panic

[asomers]

Summary

geli: fix a livelock during panic

During any kind of shutdown, kern_reboot calls geli's pre_sync event hook,
which tries to destroy all unused geli devices. But during a panic, geli
can't destroy any devices, because the scheduler is stopped, so it can't
switch threads. A livelock results, and the system never dumps core.

This commit fixes the problem by refusing to destroy any devices during
panic, used or otherwise.

PR: 246207
Sponsored By: Axcient

Test Plan

Manual testing as described in PR.

[asomers]

ping @jhb @pjd does this change look ok to you two?

[jhb]

sys/geom/eli/g_eli.c

1416 ↗

(On Diff #71402)

kib@ would probably want you to do != 0 for all of those, but I'm fine with whatever pattern is normally used here in GEOM (e.g. should it be testing the sum != 0 instead of a bunch of ||?)

[asomers]

Ok, I'll commit it kib's way. Checking the sum feels like a hack.