selftests/x86/sigreturn/64: Fix spurious failures on AMD CPUs

When I wrote the sigreturn test, I didn't realize that AMD's busted
IRET behavior was different from Intel's busted IRET behavior:

On AMD CPUs, the CPU leaks the high 32 bits of the kernel stack pointer
to certain userspace contexts.  Gee, thanks.  There's very little
the kernel can do about it.  Modify the test so it passes.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/86e7fd3564497f657de30a36da4505799eebef01.1530076529.git.luto@kernel.org


Signed-off-by: Ingo Molnar <mingo@kernel.org>