btrfs: uapi/linux/btrfs_tree.h migration, item types and defines

The BTRFS_IOC_SEARCH_TREE ioctl returns file system items directly
to userspace.  In order to decode them, full type information is required.

Create a new header, btrfs_tree to contain these since most users won't
need them.

Signed-off-by: Jeff Mahoney <[email protected]>
Reviewed-by: Liu Bo <[email protected]>
Reviewed-by: Josef Bacik <[email protected]>
Signed-off-by: David Sterba <[email protected]>

Author: jeffmahoney