genirq/ipi: Fixup checks against nr_cpu_ids

Valid CPU ids are [0, nr_cpu_ids-1] inclusive.

Fixes: 3b8e29a8 ("genirq: Implement ipi_send_mask/single()")
Fixes: f9bce791 ("genirq: Add a new function to get IPI reverse mapping")
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
Link: http://lkml.kernel.org/r/20170819095751.GB27864@avx2