irqtime: Make accounting correct on RT

vtime_account_irq and irqtime_account_irq() base checks on preempt_count()
which fails on RT because preempt_count() does not contain the softirq
accounting which is seperate on RT.

These checks do not need the full preempt count as they only operate on the
hard and softirq sections.

Use irq_count() instead which provides the correct value on both RT and non
RT kernels. The compiler is clever enough to fold the masking for !RT:

       99b:	65 8b 05 00 00 00 00 	mov    %gs:0x0(%rip),%eax
 -     9a2:	25 ff ff ff 7f       	and    $0x7fffffff,%eax
 +     9a2:	25 00 ff ff 00       	and    $0xffff00,%eax

Reported-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Tested-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20210309085727.153926793@linutronix.de