osd: change warn_interval_multiplier to uint32_t

to prevent overflow in OpTracker::check_ops_in_flight when
multiplying warn_interval_multiplier *= 2

Backport: cuttlefish, dumpling

http://tracker.ceph.com/issues/6370 fixes #6370

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