osd: Avoid pwlc spanning intervals

osd: Avoid pwlc spanning intervals

Prevent the first write to FastEC in each interval from being
a partial write to avoid the span of partial writes tracked by
pwlc from spanning intervals. This stops bugs such as 73891
where a divergent write was not removed from the log because
pwlc recorded that the shard had not participated in writes
before and after the divergent write.

Fixes: https://tracker.ceph.com/issues/73891
Signed-off-by: Bill Scales <bill_scales@uk.ibm.com>
(cherry picked from commit dc0a195b937eb175cf663d48a856f486435e987f)

Conflicts:
  src/osd/ECBackend.cc
- change assert to ceph_assert