Fix reporting of scrub limits on multi-device filesystems

[jonasmalacofilho]

On multi-device filesystems, scrub status should report "some limits set" if at least one device has a scrub limit set.

However, with btrfs-progs 6.6.3, this was being reported regardless of whether any limit actually being set:

# sudo btrfs scrub limit /more/butter
UUID: 989129d9-c96f-4d52-9d68-cbb6d9b2c499
Id  Limit       Path
--  -----  ---------
 1      -  /dev/sdc1
 2      -  /dev/sdd1

# sudo btrfs scrub status /more/butter/
UUID:             989129d9-c96f-4d52-9d68-cbb6d9b2c499
Scrub started:    Mon Jan 15 02:00:30 2024
Status:           running
Duration:         6:23:19
Time left:        0:49:08
ETA:              Mon Jan 15 09:12:57 2024
Total to scrub:   9.83TiB
Bytes scrubbed:   8.72TiB  (88.64%)
Rate:             397.47MiB/s (some device limits set)
Error summary:    no errors found

Fixes: #727

[kdave]

Thank you very much for the report and perfect pull request. Applied.

[jonasmalacofilho]

Thanks!