ARC: make futex_atomic_cmpxchg_inatomic() return bimodal

Callers of cmpxchg_futex_value_locked() in futex code expect bimodal
return value:
  !0 (essentially -EFAULT as failure)
   0 (success)

Before this patch, the success return value was old value of futex,
which could very well be non zero, causing caller to possibly take the
failure path erroneously.

Fix that by returning 0 for success

(This fix was done back in 2011 for all upstream arches, which ARC
obviously missed)

Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Michel Lespinasse <walken@google.com>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>

arch/arc/include/asm/futex.h

@@ -141,11 +141,13 @@ static inline int futex_atomic_op_inuser(int encoded_op, u32 __user *uaddr)
 
 /*
  * cmpxchg of futex (pagefaults disabled by caller)
+ * Return 0 for success, -EFAULT otherwise
  */
 static inline int
 futex_atomic_cmpxchg_inatomic(u32 *uval, u32 __user *uaddr, u32 expval,
 			      u32 newval)
 {
+	int ret = 0;
 	u32 existval;
 
 	if (!access_ok(VERIFY_WRITE, uaddr, sizeof(u32)))
@@ -155,18 +157,18 @@ futex_atomic_cmpxchg_inatomic(u32 *uval, u32 __user *uaddr, u32 expval,
 
 	__asm__ __volatile__(
 #ifdef CONFIG_ARC_HAS_LLSC
-	"1:	llock	%0, [%3]		\n"
-	"	brne	%0, %1, 3f		\n"
-	"2:	scond	%2, [%3]		\n"
+	"1:	llock	%1, [%4]		\n"
+	"	brne	%1, %2, 3f		\n"
+	"2:	scond	%3, [%4]		\n"
 	"	bnz	1b			\n"
 #else
-	"1:	ld	%0, [%3]		\n"
-	"	brne	%0, %1, 3f		\n"
-	"2:	st	%2, [%3]		\n"
+	"1:	ld	%1, [%4]		\n"
+	"	brne	%1, %2, 3f		\n"
+	"2:	st	%3, [%4]		\n"
 #endif
 	"3:	\n"
 	"	.section .fixup,\"ax\"	\n"
-	"4:	mov %0, %4	\n"
+	"4:	mov %0, %5	\n"
 	"	b   3b	\n"
 	"	.previous	\n"
 	"	.section __ex_table,\"a\"	\n"
@@ -174,14 +176,14 @@ futex_atomic_cmpxchg_inatomic(u32 *uval, u32 __user *uaddr, u32 expval,
 	"	.word   1b, 4b	\n"
 	"	.word   2b, 4b	\n"
 	"	.previous\n"
-	: "=&r"(existval)
+	: "+&r"(ret), "=&r"(existval)
 	: "r"(expval), "r"(newval), "r"(uaddr), "ir"(-EFAULT)
 	: "cc", "memory");
 
 	smp_mb();
 
 	*uval = existval;
-	return existval;
+	return ret;
 }
 
 #endif