x86/vdso: Move VDSO clocksource state tracking to callback

All architectures which use the generic VDSO code have their own storage
for the VDSO clock mode. That's pointless and just requires duplicate code.

X86 abuses the function which retrieves the architecture specific clock
mode storage to mark the clocksource as used in the VDSO. That's silly
because this is invoked on every tick when the VDSO data is updated.

Move this functionality to the clocksource::enable() callback so it gets
invoked once when the clocksource is installed. This allows to make the
clock mode storage generic.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Michael Kelley <mikelley@microsoft.com>  (Hyper-V parts)
Reviewed-by: Vincenzo Frascino <vincenzo.frascino@arm.com> (VDSO parts)
Acked-by: Juergen Gross <jgross@suse.com> (Xen parts)
Link: https://lkml.kernel.org/r/20200207124402.934519777@linutronix.de

arch/x86/entry/vdso/vma.c

@@ -38,6 +38,8 @@ struct vdso_data *arch_get_vdso_data(void *vvar_page)
 }
 #undef EMIT_VVAR
 
+unsigned int vclocks_used __read_mostly;
+
 #if defined(CONFIG_X86_64)
 unsigned int __read_mostly vdso64_enabled = 1;
 #endif
@@ -445,6 +447,8 @@ __setup("vdso=", vdso_setup);
 
 static int __init init_vdso(void)
 {
+	BUILD_BUG_ON(VCLOCK_MAX >= 32);
+
 	init_vdso_image(&vdso_image_64);
 
 #ifdef CONFIG_X86_X32_ABI