ixgbe: prefer enabling olflags rather than not disabling

ixgbe: prefer enabling olflags rather than not disabling

RTE_IXGBE_RX_OLFLAGS_ENABLE gives a hint whick keeping packet type
in RX ol_flags or not.
By default it is set to update ol_flags in RX mbuf header.
If unset it, will gain addtional performance, but will lose packet
type information.

Signed-off-by: Cunming Liang <cunming.liang@intel.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Yong Liu <yong.liu@intel.com>
Tested-by: Zhaochen Zhan <zhaochen.zhan@intel.com>