nda: React to namespace change events

Register for AC_GETDEV_CHANGED. When we receive a namespace
notification, we only create a new device if it was unconfigured. If it
was configured, generate this async event. Rely on the fact that we
reconstruct namespace to just get the data from the identify data and
call disk_resised.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D33032

(cherry picked from commit 86d3ec359a56d1b5d015718bd19ef4bda681a032)