Replace __float2half_rn with __float2half

The latter provides a consistent definition for CUDA 8.0 and 9.0.

Eigen/src/Core/arch/CUDA/PacketMathHalf.h

diff --git a/Eigen/src/Core/arch/CUDA/PacketMathHalf.h b/Eigen/src/Core/arch/CUDA/PacketMathHalf.h
index e3e45b0..8a6f209 100644
--- a/Eigen/src/Core/arch/CUDA/PacketMathHalf.h
+++ b/Eigen/src/Core/arch/CUDA/PacketMathHalf.h
@@ -231,7 +231,7 @@
 #else
   float a1 = __low2float(a);
   float a2 = __high2float(a);
-  return Eigen::half(__float2half_rn(a1 + a2));
+  return Eigen::half(__float2half(a1 + a2));
 #endif
 }
 
@@ -265,7 +265,7 @@
 #else
   float a1 = __low2float(a);
   float a2 = __high2float(a);
-  return Eigen::half(__float2half_rn(a1 * a2));
+  return Eigen::half(__float2half(a1 * a2));
 #endif
 }