crimson/os/seastore: set ExtentPlacementManager::allocated_to before rolling segments

crimson/os/seastore: set ExtentPlacementManager::allocated_to before rolling segments

There are circumstances in which a transaction that are supposed to roll the current segment
is invalidated after it finished writing and before it rolls the segment. If we don't set
ExtentPlacementManager::allocated_to in this situation, another transaction can try to write
to the old "allocated_to" position, which would cause an invalid write error

Fixes: https://tracker.ceph.com/issues/52894
Signed-off-by: Xuehan Xu <xxhdx1985126@gmail.com>