riscv: mm: init: remove _pt_ops and use pt_ops directly

Except "pt_ops", other global vars when CONFIG_XIP_KERNEL=y is defined
as below:

|foo_type foo;
|#ifdef CONFIG_XIP_KERNEL
|#define foo	(*(foo_type *)XIP_FIXUP(&foo))
|#endif

Follow the same way for pt_ops to unify the style and to simplify code.
Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Reviewed-by: Alexandre Ghiti <alex@ghiti.fr>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

arch/riscv/mm/init.c

@@ -227,12 +227,10 @@ static void __init setup_bootmem(void)
 }
 
 #ifdef CONFIG_MMU
-static struct pt_alloc_ops _pt_ops __initdata;
+static struct pt_alloc_ops pt_ops __initdata;
 
 #ifdef CONFIG_XIP_KERNEL
-#define pt_ops (*(struct pt_alloc_ops *)XIP_FIXUP(&_pt_ops))
-#else
-#define pt_ops _pt_ops
+#define pt_ops (*(struct pt_alloc_ops *)XIP_FIXUP(&pt_ops))
 #endif
 
 unsigned long riscv_pfn_base __ro_after_init;