nvd: handle namespace changes

Signal the new media size when the namespace changes size.

Approved by: re (cperciva)
Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D33032

(cherry picked from commit bd769e73d8f1d5141b1c2eb2322b4c6caed5d9e0)
(cherry picked from commit 96a7ed1e7fb41df17e783503371533cc8db98540)