[ipoib] Mask out non-QPN bits in the IPoIB destination MAC when sending

The first byte of the IPoIB MAC address is used for flags indicating
support for "connected mode".  Strip out the non-QPN bits of the first
dword when constructing the address vector for transmitted IPoIB
packets, so as not to end up passing an invalid QPN in the BTH.

src/include/gpxe/infiniband.h

@@ -30,6 +30,9 @@ FILE_LICENCE ( GPL2_OR_LATER );
 /** Broadcast QPN */
 #define IB_QPN_BROADCAST 0xffffffUL
 
+/** QPN mask */
+#define IB_QPN_MASK 0xffffffUL
+
 /** Default Infiniband partition key */
 #define IB_PKEY_DEFAULT 0xffff
 

src/include/gpxe/ipoib.h

@@ -17,9 +17,10 @@ FILE_LICENCE ( GPL2_OR_LATER );
 struct ipoib_mac {
 	/** Queue pair number
 	 *
-	 * MSB must be zero; QPNs are only 24-bit.
+	 * MSB indicates support for IPoIB "connected mode".  Lower 24
+	 * bits are the QPN.
 	 */
-	uint32_t qpn;
+	uint32_t flags__qpn;
 	/** Port GID */
 	struct ib_gid gid;
 } __attribute__ (( packed ));