[arbel] Avoid potential integer overflow when calculating memory mappings

When the area to be mapped straddles the 2GB boundary, the expression
(high+size) will overflow on the first loop iteration.  Fix by using
(end-size), which cannot underflow.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/drivers/infiniband/arbel.c

 		if ( ( low - size ) >= start ) {
 			low -= size;
 			pa = low;
-		} else if ( ( high + size ) <= end ) {
+		} else if ( high <= ( end - size ) ) {
 			pa = high;
 			high += size;
 		} else {