10132: osd: tries to set ioprio when the config option is blank

According to documentation, ioprio params will only be used if both
osd disk thread ioprio class and osd disk thread ioprio priority are
set to a non default value.

So, add a proper check and do not generate "set_disk_tp_priority(22)
Invalid argument" warning for the default settings.
Merge pull request #3245 from trociny/fix-10132

10132: osd: tries to set ioprio when the config option is blank

Reviewed-by: Sage Weil <sage@redhat.com>