btrfs: add raid stripe tree to features enabled with debug config

Until the raid stripe tree code is well enough tested and feature
complete, "hide" it behind CONFIG_BTRFS_DEBUG so only people who
want to use it are actually using it.

The scrub support may still fail some tests (btrfs/060 and up) and will
be fixed, RAID5/6 is not supported.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>