Don't consider readPreference "primary" as equivalent to 'slaveOk' in OP_QUERY find path

In db/query/find.cpp, we check if it's safe to serve the current read, but we pass the value of slaveOk based on whether there is any readPrefernce provided, rather than checking that the read preference is not "primary". This could potentially allow queries run via this path to execute against a secondary, even if they were run with the "primary" read preference.