nios2: fix __wsum type degradation in csum_fold()

csum_fold() shifts a bitwise __wsum type, which sparse reports as
"restricted __wsum degrades to integer". Sparse only checks the body of
a static inline once something instantiates it, so this went unnoticed
until a recent block layer change added the first csum_fold() caller to
a file built on this architecture.

Force cast to a __u32 for the shift, as the other architectures do.

Fixes: eea9507a69 ("nios2: Library functions")
Reported-by: kernel test robot <[email protected]>
Closes: https://lore.kernel.org/oe-kbuild-all/[email protected]/
Signed-off-by: Keith Busch <[email protected]>
Signed-off-by: Dinh Nguyen <[email protected]>

arch/nios2/include/asm/checksum.h

@@ -27,7 +27,7 @@ static inline __sum16 csum_fold(__wsum sum)
 		"add	%0, %0, r8\n"
 		"nor	%0, %0, %0\n"
 		: "=r" (sum)
-		: "r" (sum << 16), "0" (sum)
+		: "r" ((__force u32)sum << 16), "0" (sum)
 		: "r8");
 	return (__force __sum16) sum;
 }