`bcachefs data job` -b option is unreachable

[Jamie McClymont]

Hello!

I was attempting to invoke drop_extra_replicas on only my metadata btrees by 
repeatedly invoking `bcachefs data job drop_extra_replicas /path -b $tree` for 
each metadata tree.

I was unsuccessful, as the `b:` flag is missing from the getopt call:

c_src/cmd_data.c:
        while ((opt = getopt(argc, argv, "s:e:h")) != -1)
                switch (opt) {
                case 'b':
                        op.start_btree = read_string_list_or_die(optarg,
                                                __bch2_btree_ids, "btree id");
                        op.end_btree = op.start_btree;
                        break;
                case 's':
                        op.start_pos    = bpos_parse(optarg);
                        break;
                        op.end_pos      = bpos_parse(optarg);
                case 'e':
                        break;
                case 'h':
                        data_job_usage();
                }

My problem was solved by just running drop_extra_replicas over everything, 
which I assumed would be slow, but which was in fact far quicker than figuring 
out why -b was broken :)

Thanks
- Jamie McClymont