btrfs: move btrfs on-disk definitions out of ctree.h

The bulk of our on-disk definitions exist in btrfs_tree.h, which user
space can use.  Keep things consistent and move the rest of the on disk
definitions out of ctree.h into btrfs_tree.h.  Note I did have to update
all u8's to __u8, but otherwise this is a strict copy and paste.

Most of the definitions are mainly for internal use and are not
guaranteed stable public API and may change as we need. Compilation
failures by user applications can happen.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
[ reformat comments, style fixups ]
Signed-off-by: David Sterba <dsterba@suse.com>