[CORE] vmm_timer: Minor improvement to vmm_timer_timestamp()

The vmm_timer_timestamp() is the most abused API of core. It can be
called even from places where vmm_timer subsystem is not yet initialized
hence this patch adds appropriate sanity check for this function.

Signed-off-by: Anup Patel <anup@brainfault.org>
[DRIVERS] Minor fix in linux/module.h

This patch fixes KBUILD_MODNAME macro which was broken due to
recent changes.

Signed-off-by: Anup Patel <anup@brainfault.org>