rgw: RGWRadosGetOmapKeysCR uses completion return code

rgw: RGWRadosGetOmapKeysCR uses completion return code

when this operates on a nonexistent object, the osd will reject the
request with ENOENT before trying to process the subops. so Objecter
will get back a subop return code of 0, try to decode an empty
bufferlist into the result and map that subop return code to EIO

by using the AioCompletion's return code, we get the correct result of
ENOENT instead

Signed-off-by: Casey Bodley <cbodley@redhat.com>