x86/segments/64: When loadsegment(fs, ...) fails, clear the base

On AMD CPUs, a failed loadsegment currently may not clear the FS
base.  Fix it.

While we're at it, prevent loadsegment(gs, xyz) from even compiling
on 64-bit kernels.  It shouldn't be used.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
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/a084c1b93b7b1408b58d3fd0b5d6e47da8e7d7cf.1461698311.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/kernel/cpu/common.c

@@ -430,7 +430,7 @@ void load_percpu_segment(int cpu)
 #ifdef CONFIG_X86_32
 	loadsegment(fs, __KERNEL_PERCPU);
 #else
-	loadsegment(gs, 0);
+	__loadsegment_simple(gs, 0);
 	wrmsrl(MSR_GS_BASE, (unsigned long)per_cpu(irq_stack_union.gs_base, cpu));
 #endif
 	load_stack_canary_segment();