x86/vsyscall/64: Use proper accessor to update P4D entry

Writing to it directly does not work for Xen PV guests.

Fixes: 49275fe ("x86/vsyscall/64: Explicitly set _PAGE_USER in the pagetable hierarchy")
Signed-off-by: Boris Ostrovsky <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Reviewed-by: Juergen Gross <[email protected]>
Acked-by: Andy Lutomirski <[email protected]>
Cc: [email protected]
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

Author: Boris Ostrovsky

arch/x86/entry/vsyscall/vsyscall_64.c

@@ -347,7 +347,7 @@ void __init set_vsyscall_pgtable_user_bits(pgd_t *root)
 	set_pgd(pgd, __pgd(pgd_val(*pgd) | _PAGE_USER));
 	p4d = p4d_offset(pgd, VSYSCALL_ADDR);
 #if CONFIG_PGTABLE_LEVELS >= 5
-	p4d->p4d |= _PAGE_USER;
+	set_p4d(p4d, __p4d(p4d_val(*p4d) | _PAGE_USER));
 #endif
 	pud = pud_offset(p4d, VSYSCALL_ADDR);
 	set_pud(pud, __pud(pud_val(*pud) | _PAGE_USER));