[hwasan] Fix kernel instrumentation of stack.

Summary:
Kernel addresses have 0xFF in the most significant byte.
A tag can not be pushed there with OR (tag << 56);
use AND ((tag << 56) | 0x00FF..FF) instead.

Reviewers: kcc, andreyknvl

Subscribers: srhines, llvm-commits, hiraditya

Differential Revision: https://reviews.llvm.org/D42941

llvm-svn: 324691