nvme: Notify namespace changes better

When we get a namespace notification, we have to reconstrut the
namespace to get the new identification data from the namespace. For
each namespace in the AEN, we will reconstrict it before we call the
notification. We also flag it as changed for the duration of the change
callback (prior versions of the patch needed to keep track, but we no
longer do, so this bit may be removed). Note when we've seen the
namespace so we can notify when it goes away.

Co-authored-by: imp
Differential Revision: https://reviews.freebsd.org/D33032

(cherry picked from commit 20e94950c54e398049396647da36b9e2c3b639c1)