x86/asm: Simplify __smp_mb() definition

Drop the bitness ifdeffery in favor of using _ASM_SP,
which is the helper macro for the rSP register specification
for 32 and 64 bit depending on the build.

No functional changes.

Signed-off-by: Borislav Petkov <[e-mail]>
Signed-off-by: Ingo Molnar <[e-mail]>
Acked-by: Peter Zijlstra (Intel) <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

arch/x86/include/asm/barrier.h

···
 #define dma_rmb()	barrier()
 #define dma_wmb()	barrier()
 
-#ifdef CONFIG_X86_32
-#define __smp_mb()	asm volatile("lock; addl $0,-4(%%esp)" ::: "memory", "cc")
-#else
-#define __smp_mb()	asm volatile("lock; addl $0,-4(%%rsp)" ::: "memory", "cc")
-#endif
+#define __smp_mb()	asm volatile("lock; addl $0,-4(%%" _ASM_SP ")" ::: "memory", "cc")
+
 #define __smp_rmb()	dma_rmb()
 #define __smp_wmb()	barrier()
 #define __smp_store_mb(var, value) do { (void)xchg(&var, value); } while (0)