osd_types::osd_stat_sum_t: fix floor for num_objects_omap

Introduced in a130a4452e4fb159dc62fb417077d98dc9ebd621
Signed-off-by: Samuel Just <sam.just@inktank.com>
OSD: split pg stats during pg split

Fixes: #8020
Signed-off-by: Samuel Just <sam.just@inktank.com>