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/mm/numa.c

@@ -36,6 +36,12 @@ struct node_cpuid_s node_cpuid[NR_CPUS] =
  */
 u8 numa_slit[MAX_NUMNODES * MAX_NUMNODES];
 
+int __node_distance(int from, int to)
+{
+	return slit_distance(from, to);
+}
+EXPORT_SYMBOL(__node_distance);
+
 /* Identify which cnode a physical address resides on */
 int
 paddr_to_nid(unsigned long paddr)