crimson/os/seastore/transaction_manager: consider inconsistency between

crimson/os/seastore/transaction_manager: consider inconsistency between
backrefs and lbas acceptable when cleaning segments

Consider the following scene:
1. Trans.A writes the final record of Segment S, in which it overwrite
   another extent E in the same segment S;
2. Before Trans.A "complete_commit", Trans.B tries to rewrite new
   records and roll the segments, which closes Segment S;
3. Before Trans.A "complete_commit", a new cleaner Transaction C tries
   to clean the segment;

In this scenario, C might see a part of extent E's laddr space mapped
to another location within the same segment S. This is actually a valid
case.

Fixes: https://tracker.ceph.com/issues/66924
Signed-off-by: Xuehan Xu <xuxuehan@qianxin.com>