os/bluestore: fix improper offset calculation when repairing.

os/bluestore: fix improper offset calculation when repairing.

While repairing misreferenced blobs BlueStore could improperly calculate
an offset within a blob being fixed. This could happen when single
physical extent has been replaced by multiple ones - the following
pextent (if any in the current blob) would be treated with the improper offset within the blob. Offset calculation didn't account for each of that new pextents but the last one only.

Fixes: https://tracker.ceph.com/issues/51682
Signed-off-by: Igor Fedotov <ifedotov@suse.com>
(cherry picked from commit ca4b6675fc3fd2f4cadad58044c97c5bb23d5938)