Description of problem:
When doing cross-arch builds with mock, it uses qemu-user-static under the hood, and qemu-user-static lacks support for Btrfs ioctls to emulate so that btrfs(8) commands work correctly.

This is especially important for being able to do cross-arch image builds.

Version-Release number of selected component (if applicable):
5.1.0-2.fc33

How reproducible:
Always

Steps to Reproduce:

$ sudo dnf install mock qemu-user-static wget
$ sudo usermod -a -G mock $USER
$ newgrp mock
$ mock --root fedora-rawhide-armhfp --install btrfs-progs util-linux
$ mock --root fedora-rawhide-armhfp --chroot 'rm -f foo.img && dd if=/dev/zero of=foo.img bs=1G count=1 && losetup /dev/loop9 foo.img &&  mkfs.btrfs /dev/loop9 && mkdir /foo && mount /dev/loop9 /foo && btrfs subvol create /foo/subvol && umount /foo && losetup -d /dev/loop9'


Actual results:
Fails with errors like "ERROR: cannot create subvolume: Function not implemented"

Expected results:
Succeeds and creates subvolumes properly.

Additional info:
There is a patch series from a few days ago to add support for many btrfs ioctls

https://lists.gnu.org/archive/html/qemu-devel/2020-08/msg05594.html