btrfs: stop accessing ->extent_root directly

When we start having multiple extent roots we'll need to use a helper to
get to the correct extent_root.  Rename fs_info->extent_root to
_extent_root and convert all of the users of the extent root to using
the btrfs_extent_root() helper.  This will allow us to easily clean up
the remaining direct accesses in the future.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
diff --git a/fs/btrfs/disk-io.h b/fs/btrfs/disk-io.h
index baca295..e2824c6 100644
--- a/fs/btrfs/disk-io.h
+++ b/fs/btrfs/disk-io.h
@@ -103,9 +103,15 @@ static inline struct btrfs_root *btrfs_grab_root(struct btrfs_root *root)
 	return NULL;
 }
 
+static inline struct btrfs_root *btrfs_extent_root(struct btrfs_fs_info *fs_info,
+						   u64 bytenr)
+{
+	return fs_info->_extent_root;
+}
+
 static inline struct btrfs_root *btrfs_block_group_root(struct btrfs_fs_info *fs_info)
 {
-	return fs_info->extent_root;
+	return btrfs_extent_root(fs_info, 0);
 }
 
 void btrfs_put_root(struct btrfs_root *root);