[MIPS] Fix wrong checksum calculation on 64-bit MIPS

The commit 8e3d843 ([NET]: MIPS
checksum annotations and cleanups) broke 64-bit MIPS.

The problem is the commit replaces some unsigned long with __be32.  On
64bit MIPS, a __be32 (i.e. unsigned int) value is represented as a
sign-extented 32-bit value in a 64-bit argument register.  So the
address 192.168.0.1 (0xc0a80001) is passed as 0xffffffffc0a80001 to
csum_tcpudp_nofold() but the asm code in the function expects
0x00000000c0a80001, therefore it returns a wrong checksum.  Explicit
cast to unsigned long is needed to drop high 32bit.

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

include/asm-mips/checksum.h

@@ -159,7 +159,8 @@ static inline __wsum csum_tcpudp_nofold(__be32 saddr,
 #endif
 	"	.set	pop"
 	: "=r" (sum)
-	: "0" (daddr), "r"(saddr),
+	: "0" ((__force unsigned long)daddr),
+	  "r" ((__force unsigned long)saddr),
 #ifdef __MIPSEL__
 	  "r" ((proto + len) << 8),
 #else