sanitizer: Use pre-computed size of struct ustat for Linux

<sys/ustat.h> has been removed from glibc 2.28 by:

commit cf2478d53ad7071e84c724a986b56fe17f4f4ca7
Author: Adhemerval Zanella <adhemerval.zanella@linaro.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

https://bugs.llvm.org/show_bug.cgi?id=37418

Patch by H.J. Lu.

Differential Revision: https://reviews.llvm.org/D47281

llvm-svn: 333213