osd: optimize extent comparison in PrimaryLogPG

osd: optimize extent comparison in PrimaryLogPG

Improve the performance of finish_extent_cmp by using bufferlist iterators
instead of array access operator[]. This change:

- Replaces O(N) array access with O(1) iterator operations

The original implementation used array access which has O(N) complexity
for each access in bufferlist (N being the number of buffers in a bufferlist).
The new version uses iterators which wprovide O(1) increment operations,
reducing the overall function complexity from O(M*N) to O(M) with M being the
length of the bufferlist.

Fixes: https://tracker.ceph.com/issues/69014
Signed-off-by: Dongdong Tao <dongdong.tao@canonical.com>
(cherry picked from commit 60797f6862f8387be2b09ad61e198a60c55383d5)