resizeFilesystem: invoke resize2fs against the partition device, not the whole disk

[eriknordmark]

Summary

Fixes #9.

When resizeFilesystem is invoked against a real block device (DeviceTypeBlockDevice branch), it was handing the whole-disk path to resize2fs rather than the partition device path. resize2fs /dev/sda finds no filesystem at offset 0 and errors out; what was needed was resize2fs /dev/sda9 (or /dev/nvme0n1p9, /dev/mmcblk0p9, …).

Approach

Resolve the partition device path by sysfs lookup rather than hardcoding the naming convention:

func partitionDevicePath(diskPath string, partNumber int, syspath string) (string, error) {
    diskBase := filepath.Base(diskPath)
    diskSysDir := filepath.Join(syspath, "class", "block", diskBase)
    entries, _ := os.ReadDir(diskSysDir)
    for _, e := range entries {
        raw, err := os.ReadFile(filepath.Join(diskSysDir, e.Name(), "partition"))
        if err != nil { continue }
        if n, _ := strconv.Atoi(strings.TrimSpace(string(raw))); n == partNumber {
            return filepath.Join("/dev", e.Name()), nil
        }
    }
    ...
}

The kernel-named partition directory under sysfs always matches the device node under /dev, so the lookup gives us back sda9, nvme0n1p9, mmcblk0p9, etc. correctly without us having to maintain a naming-convention table.

The disk-image (DeviceTypeFile) branch is unchanged — it continues to copy the partition's bytes out to a temp file, resize that, and copy back.

Tests

New TestPartitionDevicePath covers:

• sda → sda9 (traditional convention).
• nvme0n1 → nvme0n1p9 (the "p"-prefix convention used by NVMe / eMMC / virtio-blk).
• partition number not found under sysfs (returns error).
• disk not found under sysfs (returns error).

Uses a fake-sysfs tree under t.TempDir(), same pattern as the existing TestFindDisks.

Test plan

$ go test -run '^TestShrinkFilesystem$|^TestPartitionDevicePath$' .
ok      github.com/diskfs/partitionresizer  0.816s

End-to-end against a real block device isn't unit-testable here, but the load-bearing piece is the partition-name resolution, which is now under test.

Signed-off-by: eriknordmark erik@zededa.com