install/56886: sysinst doesn't show "Add a partition" menu after deleting partitions

[Izumi Tsutsui <tsutsui%ceres.dti.ne.jp@localhost>]

>Number:         56886
>Category:       install
>Synopsis:       sysinst doesn't show "Add a partition" menu after deleting partitions
>Confidential:   no
>Severity:       serious
>Priority:       high
>Responsible:    install-manager
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Wed Jun 15 11:05:00 +0000 2022
>Originator:     Izumi Tsutsui
>Release:        NetBSD 9.99.97 2202206130350Z daily snapshot
>Organization:
>Environment:
System: NetBSD 9.2 (GENERIC)
Architecture: i386, but maybe MI issue
Machine: i386
>Description:
The sysinst inner (disklabel) partition editor doesn't show
"Add a partition" menu item even after deleting partition
or resizing partition smaller.

>How-To-Repeat:
% ftp http://nycdn.netbsd.org/pub/NetBSD-daily/HEAD/202206130350Z/images/NetBSD-9.99.97-i386.iso
% dd if=/dev/zero of=test.img bs=1m count=1024
% qemu-system-i386 -hda test.img -cdrom NetBSD-9.99.97-i386.iso

After sysinst starts:
 - a: Installation messages in English
 - a: Install NetBSD to hard disk 
 - b: Yes 
 - a: wd0 (1.0G, QEMU HARDDISK)
 - b: Master Boot Record (MBR)
 - a: This is the correct geometry
 - b: Use the entire disk
 - a: Set sizes of NetBSD partitions
 - x: Go on.  Free space 0 sec.

Here, there is no "Add a partition" menu item, maybe because
there is no free space. Maybe this is intended behaviour.
However we can never add partitions even after deleting or changing
existing partitions.

- case 1:
 - (select a: partition)
 - n: Delete partition
  -> Then the menu claims "free: 340M" but
     still no "Add a partition" menu item.

- case 2:
 - (select b: partition)
 - c:             size : 1400769 sec
 - (set 65536 sec (or a value smaller than before)
 - x: OK
  -> Then the menu still claims "free: 0B" and
     still no "Add a partition" menu item.

>Fix:
Handle dp.free_space in disklabel_delete() (and all other related
functions that update partition size) in disklabel.c properly?

---
Izumi Tsutsui