powerpc/mpic: Fix allocation of reverse-map for multi-ISU mpics

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

When using a multi-ISU MPIC, we can interrupts up to
isu_size * MPIC_MAX_ISU, not just isu_size, so allocate
the right size reverse map.

Without this, the code will constantly fallback to
a linear search.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
---

diff --git a/arch/powerpc/sysdev/mpic.c b/arch/powerpc/sysdev/mpic.c
index 4e9ccb1..7fb3ab5 100644
--- a/arch/powerpc/sysdev/mpic.c
+++ b/arch/powerpc/sysdev/mpic.c
@@ -1346,7 +1346,8 @@ struct mpic * __init mpic_alloc(struct device_node *node,
 	mpic->isu_mask = (1 << mpic->isu_shift) - 1;
 
 	mpic->irqhost = irq_alloc_host(mpic->node, IRQ_HOST_MAP_LINEAR,
-				       isu_size ? isu_size : mpic->num_sources,
+				       isu_size ? isu_size *  MPIC_MAX_ISU :
+				       mpic->num_sources,
 				       &mpic_host_ops,
 				       flags & MPIC_LARGE_VECTORS ? 2048 : 256);