Fix Debian packaging postinst partition finding

[xzhangxa]

If the root partition is bind mounted with / and another, the current postinst script (using command lsblk) will fail to find the partition: $type will be "/" only and cause the following command may find the wrong partition.

And current command forces the root partition to be ext4.

This patch fixes the two issues.

Two setups with bind mounted root partition;

Ubuntu 22.04 desktop, bind mount firefox snap:

> lsblk
nvme0n1      259:19   0 931.5G  0 disk
├─nvme0n1p1  259:20   0   243M  0 part /boot/efi
├─nvme0n1p2  259:21   0 927.5G  0 part /var/snap/firefox/common/host-hunspell
│                                      /

Windows WSL2, bind mount WSLg:

> lsblk
sdc    8:32   0     1T  0 disk /mnt/wslg/distro
                               /

Tracked-On: #8532

[shuaizhx]

@xzhangxa please add Tracked-On

[xzhangxa]

@xzhangxa please add Tracked-On

Tracked-On: #8532

[xzhangxa]

I'd like to change the code a little from using df to findmnt /. When using df if the kernel cmdline has "root=/dev/xxx" the df command will show "/" on partition "/dev/root", but we need the real hardware partition. So it won't work in this case (it's also the case that if you install a new acrn-hypervisor deb under the previous acrn hypervisor+SOS).

So lsblk fails when bind mount root partition and df fails when kernel cmdline has explicit "root=/dev/xxx".

Change to use findmnt / instead, it will always report the real hardware partition.

[yuhuanX]

start to run premerge test

[yuhuanX]

Please 'terryzouhao' or 'NanlinXie' approve this patch.