x86, UV: Complete IRQ interrupt migration in arch_enable_uv_irq()

In uv_setup_irq(), the call to create_irq() initially assigns
IRQ vectors to cpu 0. The subsequent call to
assign_irq_vector() in arch_enable_uv_irq() migrates the IRQ to
another cpu and frees the cpu 0 vector - at least it will be
freed as soon as the "IRQ move" completes.

arch_enable_uv_irq() needs to send a cleanup IPI to complete
the IRQ move. Otherwise, assignment of GRU interrupts on large
systems (>200 cpus) will exhaust the cpu 0 interrupt vectors
and initialization of the GRU driver will fail.

Signed-off-by: Jack Steiner <steiner@sgi.com>
LKML-Reference: <20090720142840.GA8885@sgi.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>