Btrfs: move the extent buffer radix tree into the fs_info

I need to create a fake tree to test qgroups and I don't want to have to setup a
fake btree_inode.  The fact is we only use the radix tree for the fs_info, so
everybody else who allocates an extent_io_tree is just wasting the space anyway.
This patch moves the radix tree and its lock into btrfs_fs_info so there is less
stuff I have to fake to do qgroup sanity tests.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Chris Mason <clm@fb.com>
diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index 7158c97..a924274 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -1659,6 +1659,10 @@
 	spinlock_t reada_lock;
 	struct radix_tree_root reada_tree;
 
+	/* Extent buffer radix tree */
+	spinlock_t buffer_lock;
+	struct radix_tree_root buffer_radix;
+
 	/* next backup root to be overwritten */
 	int backup_root_index;