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 <ubizjak@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/20240430091833.196482-5-ubizjak@gmail.com

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)