fs: btrfs: Sync btrfs_btree.h from kernel

This version includes all needed on-disk format from kernel.

Only need to modify the include headers for U-Boot, everything else is
untouched.

Also, since U-Boot btrfs is using a different endian convert timing (at
tree block read time), it needs some forced type conversion before
proper crossport.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Marek Behún <marek.behun@nic.cz>