irq_work: Map irq_work_on_queue() to irq_work_on() in !SMP

Commit 47885016 ("irq_work: Implement remote queueing") provides
irq_work_on_queue() only for SMP builds.  However, providing it simplifies
code that submits irq_work to lists of CPUs, eliminating the !SMP special
cases.  This commit therefore maps irq_work_on_queue() to irq_work_on()
in !SMP builds, but validating the specified CPU.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>