test_rgw_admin_opstate.cc: remove unused variable 'creds'

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>
test_rgw_admin_log.cc: use static_cast<>() instead of C-Style cast

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>
test_rgw_admin_log.cc: remove unused variable 'creds'

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>
rgw/rgw_metadata.h: init prefix in initialization list

For performance reasons: init 'prefix' with META_LOG_OBJ_PREFIX
in the initialization list of RGWMetadataLog instead of assigning
the value in the constructor body.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>