powerpc: Use rol32() instead of opencoding in csum_fold()

rol32(x, 16) will do the rotate using rlwinm.

No need to open code using inline assembly.
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/794337eff7bb803d2c4e67d9eee635390c4c48fe.1646812553.git.christophe.leroy@csgroup.eu

arch/powerpc/include/asm/checksum.h

@@ -38,14 +38,15 @@ extern __wsum csum_and_copy_to_user(const void *src, void __user *dst,
  */
 static inline __sum16 csum_fold(__wsum sum)
 {
-	unsigned int tmp;
-
-	/* swap the two 16-bit halves of sum */
-	__asm__("rlwinm %0,%1,16,0,31" : "=r" (tmp) : "r" (sum));
-	/* if there is a carry from adding the two 16-bit halves,
-	   it will carry from the lower half into the upper half,
-	   giving us the correct sum in the upper half. */
-	return (__force __sum16)(~((__force u32)sum + tmp) >> 16);
+	u32 tmp = (__force u32)sum;
+
+	/*
+	 * swap the two 16-bit halves of sum
+	 * if there is a carry from adding the two 16-bit halves,
+	 * it will carry from the lower half into the upper half,
+	 * giving us the correct sum in the upper half.
+	 */
+	return (__force __sum16)(~(tmp + rol32(tmp, 16)) >> 16);
 }
 
 static inline u32 from64to32(u64 x)