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

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

Fixes: 49275fef

 ("x86/vsyscall/64: Explicitly set _PAGE_USER in the pagetable hierarchy")
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Juergen Gross <jgross@suse.com>
Acked-by: Andy Lutomirski <luto@kernel.org>
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/20180319143154.3742-1-boris.ostrovsky@oracle.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>