btrfs: open code btrfs_ordered_inode_tree in btrfs_inode

The structure btrfs_ordered_inode_tree is used only in one place, in
btrfs_inode. The structure itself has a 4 byte hole which is wasted
space.

Move the btrfs_ordered_inode_tree members to btrfs_inode with a common
prefix 'ordered_tree_' where the hole can be utilized and shrink inode
size.

Signed-off-by: David Sterba <dsterba@suse.com>