libceph: fix decoding of pgids

In 4f6a7e5e we effectively dropped support
for the legacy encoding for the OSDMap and incremental.  However, we didn't
fix the decoding for the pgid.

Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Yehuda Sadeh <yehuda@inktank.com>