rgw_file: return common_prefixes in lexical order

rgw_file: return common_prefixes in lexical order

Since inception RGWReaddirRequest has sent all leaf objects first
(in lexical order), then common_prefixes (in lexical order). In
hindsight, an overall listing could trivially be returned out of
lexical order, which can cause continued listing of large, mixed
directories to fail.

RCA by Dan Gryniewicz.

Fixes: https://tracker.ceph.com/issues/48410
Signed-off-by: Matt Benjamin <mbenjamin@redhat.com>
(cherry picked from commit e561e98e5cca2678854e01c990f95e474022b7ed)

Conflicts:
        src/rgw/rgw_file.h