osd/OSD: fix build_past_intervals_parallel

osd/OSD: fix build_past_intervals_parallel

We may be only calculating older past intervals and have a valid
history.same_interval_since value, in which case the local
same_interval_since value will end at the newest old interval we had to
generate.

Same as 0830275b39afc408573c2c468fa3b0e7b1d5cc4e change in generate_past_intervals()
This was introduced by 70316541bbb115d9a35954bfba373cf1dc084b7e.

Signed-off-by: David Zafman <dzafman@redhat.com>
(cherry picked from commit 0fd674bbf0c17a673be40123645adee3d64375a0)