When the sector size changes, we assume it's new media. When the
mediasize changes, we'll just resize the disk (we get called for both
events). When neither have changed, don't call either.
Some NVMe drives (but not all) post a async event on page 4 with the
sector size changes via a FORMAT command. We'll notice the new media
right away, rather than the next device open. As a practical effect,
this just means that certain geom operations will see it sooner. Since
most drive interaction goes through open, that will catch those drives
that do not post this event well enough.
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