iommu/iova: Fix module config properly

IOMMU_IOVA is intended to be an optional library for users to select as
and when they desire. Since it can be a module now, this means that
built-in code which has chosen not to select it should not fail to link
if it happens to have selected as a module by someone else. Replace
IS_ENABLED() with IS_REACHABLE() to do the right thing.

CC: Thierry Reding <thierry.reding@gmail.com>
Reported-by: John Garry <john.garry@huawei.com>
Fixes: 15bbdec ("iommu: Make the iova library a module")
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Reviewed-by: Thierry Reding <treding@nvidia.com>
Link: https://lore.kernel.org/r/548c2f683ca379aface59639a8f0cccc3a1ac050.1663069227.git.robin.murphy@arm.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>

Author: rmurphy-arm

include/linux/iova.h

@@ -75,7 +75,7 @@ static inline unsigned long iova_pfn(struct iova_domain *iovad, dma_addr_t iova)
 	return iova >> iova_shift(iovad);
 }
 
-#if IS_ENABLED(CONFIG_IOMMU_IOVA)
+#if IS_REACHABLE(CONFIG_IOMMU_IOVA)
 int iova_cache_get(void);
 void iova_cache_put(void);