x86/irq: Rename VECTOR_UNDEFINED to VECTOR_UNUSED

VECTOR_UNDEFINED is a misnomer. The vector is defined, but unused.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Jiang Liu <jiang.liu@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Bjorn Helgaas <bhelgaas@google.com>
Link: http://lkml.kernel.org/r/20150802203609.477282494@linutronix.de


Signed-off-by: Thomas Gleixner <tglx@linutronix.de>