Fix the inode count in pct-shrink-rootfs

GNU df refuses -i together with --output, so the script stopped right
after mounting the container ("options -i and --output are mutually
exclusive"). The iused field needs no -i: use df --output=iused.

The df stub in the tests accepted the combination and hid the bug; it now
rejects it the way GNU df does.

Generated with AI

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

scripts/pct-shrink-rootfs

@@ -133,7 +133,7 @@ main() {
   mounted=1
   pct mount "$ctid"
   used=$(df -B1 --output=used "$root" | tail -n1 | tr -d ' ')
-  iused=$(df -i --output=iused "$root" | tail -n1 | tr -d ' ')
+  iused=$(df --output=iused "$root" | tail -n1 | tr -d ' ')
   unmount_or_die
   [[ $used =~ ^[0-9]+$ && $iused =~ ^[0-9]+$ ]] || die "cannot read df output"