arm64: vdso: constify vm_special_mapping used for aarch32 vectors page

The vm_special_mapping spec which is used for aarch32 vectors page is
never modified, so mark it as const.
Acked-by: NMark Rutland <mark.rutland@arm.com>
Signed-off-by: NJisheng Zhang <jszhang@marvell.com>
Signed-off-by: NWill Deacon <will.deacon@arm.com>

arch/arm64/kernel/vdso.c

@@ -88,7 +88,7 @@ int aarch32_setup_vectors_page(struct linux_binprm *bprm, int uses_interp)
 {
 	struct mm_struct *mm = current->mm;
 	unsigned long addr = AARCH32_VECTORS_BASE;
-	static struct vm_special_mapping spec = {
+	static const struct vm_special_mapping spec = {
 		.name	= "[vectors]",
 		.pages	= vectors_page,