kubelet: fix disk space check on btrfs (issue 47046)

This commit fixes the warning messages reported by kubelet when
checking for the disk space on a btrfs `/` which has `/var/lib/kubelet`
inside of a btrfs sub-volume.

This fix follows the same principle adopted to fix issue kubernetes#38337 with
commit dc8b6cc.

This commit fixes issue 47046.

Signed-off-by: Flavio Castelli <fcastelli@suse.com>