x86/percpu: Fix operand constraint modifier in __raw_cpu_write()

__raw_cpu_write() with !USE_X86_SEG_SUPPORT config uses read/write
operand constraint modifier "+" for its memory location. This signals
the compiler that the location is both read and written by the asm.
This is not true, because MOV insn only writes to the output.

Correct the modifier to "=" to inform the compiler that the memory
location is only written to. This also prevents the compiler from
value tracking the undefined value from the uninitialized memory.

Signed-off-by: Uros Bizjak <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Josh Poimboeuf <[email protected]>
Cc: Linus Torvalds <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: ubizjak

arch/x86/include/asm/percpu.h

@@ -178,7 +178,7 @@ do {									\
 		(void)pto_tmp__;					\
 	}								\
 	asm qual(__pcpu_op2_##size("mov", "%[val]", __percpu_arg([var])) \
-	    : [var] "+m" (__my_cpu_var(_var))				\
+	    : [var] "=m" (__my_cpu_var(_var))				\
 	    : [val] __pcpu_reg_imm_##size(pto_val__));			\
 } while (0)