x86: vsyscall: Use NULL instead 0 for a pointer argument

This patch silences the following sparse warning:
arch/x86/kernel/vsyscall_64.c:250:34:
       warning: Using plain integer as NULL pointer

Signed-off-by: Emil Goode <emilgoode@gmail.com>
Acked-by: Andy Lutomirski <luto@amacapital.net>
Cc: john.stultz@linaro.org
Link: http://lkml.kernel.org/r/1333306084-3776-1-git-send-email-emilgoode@gmail.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>