[infiniband] Centralise assumption of 2048-byte payloads

IPoIB and the SMA have separate constants for the packet size to be
used to I/O buffer allocations.  Merge these into the single
IB_MAX_PAYLOAD_SIZE constant.

(Various other points in the Infiniband stack have hard-coded
assumptions of a 2048-byte payload; we don't currently support
variable MTUs.)

src/net/infiniband/ib_sma.c

@@ -361,7 +361,7 @@ static void ib_sma_refill_recv ( struct ib_sma *sma ) {
 	while ( sma->qp->recv.fill < IB_SMA_NUM_RECV_WQES ) {
 
 		/* Allocate I/O buffer */
-		iobuf = alloc_iob ( IB_SMA_PAYLOAD_LEN );
+		iobuf = alloc_iob ( IB_MAX_PAYLOAD_SIZE );
 		if ( ! iobuf ) {
 			/* Non-fatal; we will refill on next attempt */
 			return;