preempt: Cleanup the macro maze a bit

Make the macro maze consistent and prepare it for adding the RT variant for
BH accounting.

 - Use nmi_count() for the NMI portion of preempt count
 - Introduce in_hardirq() to make the naming consistent and non-ambiguos
 - Use the macros to create combined checks (e.g. in_task()) so the
   softirq representation for RT just falls into place.
 - Update comments and move the deprecated macros aside

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Link: https://lore.kernel.org/r/20201113141733.864469886@linutronix.de