MIPS: Fix inline asm input/output type mismatch in checksum.h used with Clang

Fix the following build error when make M=samples/bpf used with Clang:

  CLANG-bpf  samples/bpf/sockex2_kern.o
In file included from samples/bpf/sockex2_kern.c:7:
In file included from ./include/uapi/linux/if_tunnel.h:7:
In file included from ./include/linux/ip.h:16:
In file included from ./include/linux/skbuff.h:28:
In file included from ./include/net/checksum.h:22:
./arch/mips/include/asm/checksum.h:161:9: error: unsupported inline asm: input with type 'unsigned long' matching output with type '__wsum' (aka 'unsigned int')
        : "0" ((__force unsigned long)daddr),
               ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 error generated.

This is a known issue on MIPS [1], the changed code can be compiled
successfully by both GCC and Clang.

[1] https://lore.kernel.org/linux-mips/CAG_fn=W0JHf8QyUX==+rQMp8PoULHrsQCa9Htffws31ga8k-iw@mail.gmail.com/Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>

arch/mips/include/asm/checksum.h

@@ -130,6 +130,8 @@ static inline __wsum csum_tcpudp_nofold(__be32 saddr, __be32 daddr,
 					__u32 len, __u8 proto,
 					__wsum sum)
 {
+	unsigned long tmp = (__force unsigned long)sum;
+
 	__asm__(
 	"	.set	push		# csum_tcpudp_nofold\n"
 	"	.set	noat		\n"
@@ -157,7 +159,7 @@ static inline __wsum csum_tcpudp_nofold(__be32 saddr, __be32 daddr,
 	"	addu	%0, $1		\n"
 #endif
 	"	.set	pop"
-	: "=r" (sum)
+	: "=r" (tmp)
 	: "0" ((__force unsigned long)daddr),
 	  "r" ((__force unsigned long)saddr),
 #ifdef __MIPSEL__
@@ -167,7 +169,7 @@ static inline __wsum csum_tcpudp_nofold(__be32 saddr, __be32 daddr,
 #endif
 	  "r" ((__force unsigned long)sum));
 
-	return sum;
+	return (__force __wsum)tmp;
 }
 #define csum_tcpudp_nofold csum_tcpudp_nofold