btrfs: get mapping tree directly from fsinfo in find_first_block_group

We already have an fs_info in our function parameters, there's no need
to do the maths again and get fs_info from the extent_root just to get
the mapping_tree.

Instead directly grab the mapping_tree from fs_info.
Reviewed-by: NNikolay Borisov <nborisov@suse.com>
Signed-off-by: NJohannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>

fs/btrfs/block-group.c

@@ -1566,7 +1566,7 @@ static int find_first_block_group(struct btrfs_fs_info *fs_info,
 			struct extent_map_tree *em_tree;
 			struct extent_map *em;
 
-			em_tree = &root->fs_info->mapping_tree;
+			em_tree = &fs_info->mapping_tree;
 			read_lock(&em_tree->lock);
 			em = lookup_extent_mapping(em_tree, found_key.objectid,
 						   found_key.offset);