rgw: merge manifests correctly when there's prefix override

rgw: merge manifests correctly when there's prefix override

Fixes: #11622
Backport: hammer, firefly

Prefix override happens in a manifest when a rados object does not
conform to the generic prefix set on the manifest. When merging
manifests (specifically being used in multipart objects upload), we need
to check if the rule that we try to merge has a prefix that is the same
as the previous rule. Beforehand we checked if both had the same
override_prefix setting, but that might not apply as both manifests
might have different prefixes.

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