btrfs: handle space_info setting of bg in btrfs_add_bg_to_space_info

We previously had the pattern of

	btrfs_update_space_info(all, the, bg, fields, &space_info);
	link_block_group(bg);
	bg->space_info = space_info;

Now that we're passing the bg into btrfs_add_bg_to_space_info we can do
the linking in that function, transforming this to simply

	btrfs_add_bg_to_space_info(fs_info, bg);

and put the link_block_group() and bg->space_info assignment directly in
btrfs_add_bg_to_space_info.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>