x86/nmi/64: Minor asm simplification

Replace LEA; MOV with an equivalent SUB.  This saves one
instruction.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Reviewed-by: Steven Rostedt <rostedt@goodmis.org>
Cc: Borislav Petkov <bp@suse.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/entry/entry_64.S

@@ -1420,8 +1420,7 @@ nested_nmi:
 	 * Modify the "iret" frame to point to repeat_nmi, forcing another
 	 * iteration of NMI handling.
 	 */
-	leaq	-1*8(%rsp), %rdx
-	movq	%rdx, %rsp
+	subq	$8, %rsp
 	leaq	-10*8(%rsp), %rdx
 	pushq	$__KERNEL_DS
 	pushq	%rdx