btrfs/057: Update test case to keep subvolume in any kernel version

btrfs/057: Update test case to keep subvolume in any kernel version

Since commit a79a464d5675 ("btrfs: Allow rmdir(2) to delete an empty
subvolume"), rm -r can delete a subvolume too.

This test assumes that rm -r does not delete a subvolume.
Currently the commit does not affect the test since qgroup items
still exist after subvolume deletion, but we plan to change the
behavior and remove them along with subvolume deletion.

So update the test and keep subvolume (and qgroup item) in any kernel
version.

Signed-off-by: Misono Tomohiro <misono.tomohiro@jp.fujitsu.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>