osd: be_compare_scrubmaps uses incorrect j iterator

The code moved from be_select_auth_object to be_compare_scrubmaps
74bd8708dfbfd3c8e7ba3f41d8534609dcbc1237 but the j iterator is use
differently although it has the same type. Use map.begin() as a
fallback instead.

http://tracker.ceph.com/issues/10408 Fixes: #10408

Signed-off-by: Loic Dachary <ldachary@dachary.org>