uapi: add metadata flag to xdp flags

Second approach to inform driver about metadata. Let user decide if
metadata should be supported or not.

Add this flag to allow user to inform driver that metadata is used.
Set flag is sent to driver via exsisting ndo_bpf call in flag field.

Signed-off-by: Michal Swiatkowski <michal.swiatkowski@intel.com>

include/uapi/linux/if_link.h

@@ -1172,11 +1172,13 @@ enum {
 #define XDP_FLAGS_DRV_MODE		(1U << 2)
 #define XDP_FLAGS_HW_MODE		(1U << 3)
 #define XDP_FLAGS_REPLACE		(1U << 4)
+#define XDP_FLAGS_USE_METADATA		(1U << 5)
 #define XDP_FLAGS_MODES			(XDP_FLAGS_SKB_MODE | \
 					 XDP_FLAGS_DRV_MODE | \
 					 XDP_FLAGS_HW_MODE)
 #define XDP_FLAGS_MASK			(XDP_FLAGS_UPDATE_IF_NOEXIST | \
-					 XDP_FLAGS_MODES | XDP_FLAGS_REPLACE)
+					 XDP_FLAGS_MODES | XDP_FLAGS_REPLACE | \
+					 XDP_FLAGS_USE_METADATA)
 
 /* These are stored into IFLA_XDP_ATTACHED on dump. */
 enum {