common: do not set ioprio if pid is not set

Signed-off-by: Loic Dachary <loic-201408@dachary.org>
common: set_ioprio debug message including pid

Signed-off-by: Loic Dachary <loic-201408@dachary.org>
osd: log error if set_ioprio fails to parse class

Signed-off-by: Loic Dachary <loic-201408@dachary.org>
common: ceph_ioprio_string_to_class always returns -EINVAL

The l string is always empty because std::transform needs a
pre-allocated string. Replace with the in-place version. Add unit tests.

http://tracker.ceph.com/issues/9677 Fixes: #9677

Signed-off-by: Loic Dachary <loic-201408@dachary.org>
Merge pull request #2654 from dachary/wip-9677-ioprio-class

common: ceph_ioprio_string_to_class always returns -EINVAL

Reviewed-by: Sage Weil <sage@redhat.com>