bsdinstall partedit: Apply changes from scripted installs

I got a check inverted in a previous cleanup commit and as a result
partedit was only applying GEOM changes (and generating an /etc/fstab)
if it got an error reading the current GEOM mesh. Instead, it needed
to do those actions if it succeeded in reading the mesh.

The lack of /etc/fstab meant that bsdinstall mount didn't mount
anything in a scripted install.

PR: 273723
Reported by: Andrey Fesenko <andrey@bsdnir.info>
Reported by: Michal Nowak <mnowak@startmail.com>
Reviewed by: cognet, brooks
Fixes: 230990991965 bsdinstall: Handle errors from geom_gettree.
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D42236