powerpc/irq: Always free duplicate IRQ_LEGACY hosts

Since kmem caches are allocated before init_IRQ as noted in 3af259d1
(powerpc: Radix trees are available before init_IRQ), we now call
kmalloc in all cases and can can always call kfree if we are asked
to allocate a duplicate or conflicting IRQ_HOST_MAP_LEGACY host.

Signed-off-by: Milton Miller <miltonm@bga.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>