oneplus 6 userdata not expanded during install
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### Describe your issue
After installing postmarketOS onto a Oneplus 6, the partition layout leaves close to no space for installing additional apps:
```
Filesystem Size Used Available Use% Mounted on
/dev/dm-1 2.1G 2.0G 10.1M 99% /
/dev/dm-0 221.1M 56.4M 152.9M 27% /boot
tmpfs 1.5G 2.1M 1.5G 0% /run
dev 10.0M 0 10.0M 0% /dev
shm 3.7G 0 3.7G 0% /dev/shm
tmpfs 4.0M 0 4.0M 0% /sys/fs/cgroup
tmpfs 756.0M 56.0K 755.9M 0% /run/user/10000
```
sda17 looks to be the main partition and lsblk shows the full size:
```
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINTS
sda 8:0 0 116.1G 0 disk
├─sda1 8:1 0 8K 0 part
├─sda2 8:2 0 32M 0 part
├─sda3 8:3 0 1M 0 part
├─sda4 8:4 0 1M 0 part
├─sda5 8:5 0 512K 0 part
├─sda6 8:6 0 512K 0 part
├─sda7 8:7 0 256M 0 part
├─sda8 8:8 0 10M 0 part
├─sda9 8:9 0 10M 0 part
├─sda10 8:10 0 7.9M 0 part
├─sda11 8:11 0 15.8M 0 part
├─sda12 8:12 0 512K 0 part
├─sda13 8:13 0 2.8G 0 part
├─sda14 8:14 0 2.8G 0 part
├─sda15 8:15 0 100M 0 part
├─sda16 259:0 0 100M 0 part
└─sda17 259:1 0 110G 0 part
```
resize2fs does not seem to work:
```
oneplus:~$ sudo resize2fs /dev/dm-1
resize2fs 1.47.0 (5-Feb-2023)
The filesystem is already 571648 (4k) blocks long. Nothing to do!
oneplus:~$ sudo resize2fs /dev/sda17
resize2fs 1.47.0 (5-Feb-2023)
resize2fs: Resource busy while trying to open /dev/sda17
Couldn't find valid filesystem superblock.
```
#### What's the expected behaviour?
The full 110Gb of data should be available to pmOS.
#### What's the current behaviour?
Only the operating system fits onto the disk, there is no room for new applications to be installed.
#### How to reproduce your issue?
I've installed manually using pmbootstrap and following the documented steps.
```
pmbootstrap init
pmbootstrap install
fastboot erase dtbo
pmbootstrap flasher flash_rootfs --partition userdata
pmbootstrap flasher flash_kernel
```
### What device are you using?
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Oneplus 6
### On what postmarketOS version did you encounter the issue?
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* [x] edge (`master` branch)
* [ ] `v23.06`
* [ ] `v22.12` (Supported until 2023-07-06)
* [x] I confirm that the issue still is present after running `sudo apk upgrade -a`
### On what environment did you encounter the issue?
#### Environments
* [x] GNOME Shell on Mobile
* [ ] Phosh
* [ ] Plasma Mobile
* [ ] Sxmo (Wayland/Sway) **Please post the output of `sxmo_version.sh`**
* [ ] Other: **Please fill out**
### How did you get postmarketOS image?
* [ ] from https://images.postmarketos.org
* [x] I built it using pmbootstrap
* [ ] It was preinstalled on my device
### What's the build date of the image? (in yyyy-mm-dd format)
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can be left empty if unsure
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The images were built manually using pmbootstrap on 2023-07-25
### Additional information
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issue