nda: fix setting of unmappedio flag

The upstream refactoring of ndaregister() to split out ndasetgeom()
accidentally used an uninitialed variable to decide whether or not
to set DISKFLAG_UNMAPPED_BIO. Fix this by moving that portion of
ndasetgeom() back up to ndaregister(). The check for PIM_UNMAPPED
is not really needed because nvme devices always have that set,
so it cannot change in the other path that ndasetgeom() is now called.

Approved by: re (cperciva)
Reviewed by: cperciva, gallatin, imp
Fixes: dffd882d12d2a71aca464f48209ec9ae6f393b15
Sponsored by: Netflix
MFC After: 1 minute
(cherry picked from commit 2b4dbad2db5766294ee97bb96228ec6826a9e7c3)
(cherry picked from commit e271f9327f46250c9043c29c86e943d53080bf2a)