btrfs: only mark bg->needs_free_space if free space tree is on

If we attempt to create a free space tree while any block groups have
needs_free_space set, we will double add the new free space item
and hit EEXIST. Previously, we only created the free space tree on a new
mount, so we never hit the case, but if we try to create it on a
remount, such block groups could exist and trip us up.

We don't do anything with this field unless the free space tree is
enabled, so there is no harm in not setting it.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Boris Burkov <boris@bur.io>
Signed-off-by: David Sterba <dsterba@suse.com>