osd_types::object_stat_sum_t: fix add/sub for num_objects_omap

Introduced in a130a4452e4fb159dc62fb417077d98dc9ebd621
Signed-off-by: Samuel Just <sam.just@inktank.com>
Merge pull request #1697 from ceph/wip-num_objects_omap

osd_types::object_stat_sum_t: fix add/sub for num_objects_omap

Reviewed-by: Sage Weil <sage@inktank.com>