rgw: fix multipart get part when count==1

rgw: fix multipart get part when count==1

the RGWObjManifest for multipart uploads is subtly different when
there's only a single part. in that case, get_cur_part_id() for the
final rule returns 1 where it otherwise returns (parts_count + 1)

this caused two problems:
* we returned a parts_count of 0 instead 1, and
* the do-while loop got stuck in an infinite loop expecting the last
  rule's part id to be higher than the requested part id

Fixes: https://tracker.ceph.com/issues/66705
Signed-off-by: Casey Bodley <cbodley@redhat.com>