bug#29674: Ceph creates Btrfs subvolumes on Btrfs during tests.

[Rutger Helling]

Hey Guix,

I was surprised to learn that Ceph creates Btrfs subvolumes during its 
tests.
This is problematic because on Btrfs regular users can create 
subvolumes, but they cannot delete them.
This means I had to manually delete the following subvolumes as root:

ID 1744 gen 819598 top level 257 path 
tmp/guix-build-ceph-12.0.2.drv-0/td/t-7201/dev/osd0/current
ID 1745 gen 819598 top level 257 path 
tmp/guix-build-ceph-12.0.2.drv-0/td/t-7201/dev/osd0/snap_1
ID 1746 gen 819564 top level 257 path 
tmp/guix-build-ceph-12.0.2.drv-0/td/t-7201/dev/osd0/test_subvol
ID 1747 gen 819564 top level 257 path 
tmp/guix-build-ceph-12.0.2.drv-0/td/t-7201/dev/osd0/sync_snap_test
ID 1748 gen 819564 top level 257 path 
tmp/guix-build-ceph-12.0.2.drv-0/td/t-7201/dev/osd0/async_snap_test
ID 1749 gen 819598 top level 257 path 
tmp/guix-build-ceph-12.0.2.drv-0/td/t-7201/dev/osd1/current
ID 1750 gen 819598 top level 257 path 
tmp/guix-build-ceph-12.0.2.drv-0/td/t-7201/dev/osd1/snap_1
ID 1751 gen 819564 top level 257 path 
tmp/guix-build-ceph-12.0.2.drv-0/td/t-7201/dev/osd1/test_subvol
ID 1752 gen 819564 top level 257 path 
tmp/guix-build-ceph-12.0.2.drv-0/td/t-7201/dev/osd1/sync_snap_test
ID 1753 gen 819564 top level 257 path 
tmp/guix-build-ceph-12.0.2.drv-0/td/t-7201/dev/osd1/async_snap_test
ID 1754 gen 819598 top level 257 path 
tmp/guix-build-ceph-12.0.2.drv-0/td/t-7201/dev/osd2/current
ID 1755 gen 819598 top level 257 path 
tmp/guix-build-ceph-12.0.2.drv-0/td/t-7201/dev/osd2/snap_1
ID 1756 gen 819564 top level 257 path 
tmp/guix-build-ceph-12.0.2.drv-0/td/t-7201/dev/osd2/test_subvol
ID 1757 gen 819564 top level 257 path 
tmp/guix-build-ceph-12.0.2.drv-0/td/t-7201/dev/osd2/sync_snap_test
ID 1758 gen 819564 top level 257 path 
tmp/guix-build-ceph-12.0.2.drv-0/td/t-7201/dev/osd2/async_snap_test