rgw: fix listing of multipart upload parts

Fixes: #7169

There are two issues here. One is that we may return more entries than
we should (as specified by max_parts). Second issue is that the
NextPartNumberMarker is set incorrectly. Both of these issues mainly
affect uploads with > 1000 parts, although can be triggered with less
than that.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>