osd: backfill and acting are not disjoint in compat_mode

Restore the code that was changed by
92cfd370395385ca5537b5bc72220934c9f09026 when compat_mode is true.

http://tracker.ceph.com/issues/9696 Fixes: #9696

Signed-off-by: Loic Dachary <loic-201408@dachary.org>