resizing dump device while dumping panics system
While playing around trying to reproduce #3553, I happened to resize my dump device (increasing its size) while savecore -L was running. We crashed immediately.
While it's a particularly silly thing to have done, we should ideally not crash.
We're crashing under zvol_dumpio due to a NULL pointer dereference, addr=20, which actually matches what I may have seen in #3553 while resizing the device then reboot -d'ing (perhaps before a txg flushed, in that case? A complete guess based on a briefly seen panic message, what could be more reliable!).
Given addr=20, and the offset in zvol_dumpio at which we crash, I believe that list_head(&zv->zv_extents) == NULL.
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