btrfs: test delayed subvolume deletion on mount and remount

btrfs: test delayed subvolume deletion on mount and remount

Test that subvolume deletion is resumed on RW mounts, that it is not
performed on RO mounts and that after remounting a filesystem from RO
to RW mode, it is also performed.

This triggers a regression introduced in kernel 5.11 which is fixed
by a patch that has the following subject:

   "btrfs: fix subvolume/snapshot deletion not triggered on mount"

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>