Current blivet didn't invoke the default subvolume during
brtfs mounting, it lead to the higher level subvolume cannot be removed.
Solution:
Backport upstream patch to fix this issue.
* 0b672ff4 Make sure we mount the top level subvolume
when mounting btrfs
Link: 0b672ff452
Reference: bsn#313
Change-Id: If96ad4b7565a332f339734c939e5cf657627d20b
Signed-off-by: Chenxi Mao <chenxi.mao@suse.com>
