btrfs: remove bytes_used argument from btrfs_make_block_group()

The only caller of btrfs_make_block_group() always passes 0 as the value
for the bytes_used argument, so remove it.

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