fs: btrfs: Add btrfs_tree.h and ctree.h from Linux (and modified)

Add btrfs_tree.h and ctree.h from Linux which contains constants
and structures for the BTRFS filesystem.

Signed-off-by: Marek Behun <marek.behun@nic.cz>

 create mode 100644 fs/btrfs/btrfs_tree.h
 create mode 100644 fs/btrfs/ctree.h