ia64: export node_distance function

The numa_slit variable used by node_distance is available to a
module as long as it is linked at compile-time. However, it is
not available to loadable modules. Leading to errors such as:

  ERROR: "numa_slit" [drivers/nvme/host/nvme-core.ko] undefined!

The error above is caused by the nvme multipath code that makes
use of node_distance for its path calculation. When the patch was
added, the lightnvm subsystem would select nvme and always compile
it in, leading to the node_distance call to always succeed.
However, when this requirement was removed, nvme could be compiled
in as a module, which exposed this bug.

This patch extracts node_distance to a function and exports it.
Since ACPI is depending on node_distance being a simple lookup to
numa_slit, the previous behavior is exposed as slit_distance and its
users updated.

Fixes: f333444708 "nvme: take node locality into account when selecting a path"
Fixes: 73569e1103 "lightnvm: remove dependencies on BLK_DEV_NVME and PCI"
Signed-off-by: Matias Bjøring <mb@lightnvm.io>
Signed-off-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/ia64/kernel/acpi.c

@@ -578,8 +578,8 @@ void __init acpi_numa_fixup(void)
 	if (!slit_table) {
 		for (i = 0; i < MAX_NUMNODES; i++)
 			for (j = 0; j < MAX_NUMNODES; j++)
-				node_distance(i, j) = i == j ? LOCAL_DISTANCE :
-							REMOTE_DISTANCE;
+				slit_distance(i, j) = i == j ?
+					LOCAL_DISTANCE : REMOTE_DISTANCE;
 		return;
 	}
 
@@ -592,7 +592,7 @@ void __init acpi_numa_fixup(void)
 			if (!pxm_bit_test(j))
 				continue;
 			node_to = pxm_to_node(j);
-			node_distance(node_from, node_to) =
+			slit_distance(node_from, node_to) =
 			    slit_table->entry[i * slit_table->locality_count + j];
 		}
 	}