sanitizer: Use pre-computed size of struct ustat for Linux
<sys/ustat.h> has been removed from glibc 2.28 by:
Author: Adhemerval Zanella <firstname.lastname@example.org>
Date: Sun Mar 18 11:28:59 2018 +0800
Deprecate ustat syscall interface
This patch uses pre-computed size of struct ustat for Linux to fix
Patch by H.J. Lu.
Differential Revision: https://reviews.llvm.org/D47281