btrfs/301: fix hardcoded subvolids

Hardcoded subvolids break test runs with no free-space-tree, so change
the test to use _btrfs_get_subvolid instead of assuming 256, 257, etc...

Signed-off-by: Boris Burkov <boris@bur.io>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>
1 file changed