[ALTERNATE PATCH] memblock: fix min_low_pfn/max_low_pfn build errors

[Randy Dunlap]

Export min_low_pfn & max_low_pfn in mm/memblock.c to fix build errors
on arch/microblaze/ and arch/ia64/: (e.g.)

  ERROR: "max_low_pfn" [drivers/rpmsg/virtio_rpmsg_bus.ko] undefined!
  ERROR: "min_low_pfn" [drivers/rpmsg/virtio_rpmsg_bus.ko] undefined!
  ERROR: "max_low_pfn" [drivers/mtd/spi-nor/spi-nor.ko] undefined!
  ERROR: "min_low_pfn" [drivers/mtd/spi-nor/spi-nor.ko] undefined!
  ERROR: "min_low_pfn" [drivers/mtd/nand/raw/nand.ko] undefined!
  ERROR: "max_low_pfn" [drivers/mtd/nand/raw/nand.ko] undefined!
  ERROR: "max_low_pfn" [drivers/rapidio/devices/rio_mport_cdev.ko] undefined!
  ERROR: "min_low_pfn" [drivers/rapidio/devices/rio_mport_cdev.ko] undefined!
  ERROR: "min_low_pfn" [drivers/hwtracing/intel_th/intel_th_msu.ko] undefined!
  ERROR: "max_low_pfn" [drivers/hwtracing/intel_th/intel_th_msu.ko] undefined!
  ERROR: "min_low_pfn" [drivers/crypto/cavium/nitrox/n5pf.ko] undefined!
  ERROR: "max_low_pfn" [drivers/crypto/cavium/nitrox/n5pf.ko] undefined!
  ERROR: "max_low_pfn" [drivers/md/dm-integrity.ko] undefined!
  ERROR: "min_low_pfn" [drivers/md/dm-integrity.ko] undefined!
  ERROR: "max_low_pfn" [crypto/tcrypt.ko] undefined!
  ERROR: "min_low_pfn" [crypto/tcrypt.ko] undefined!

In both arches, these variables are referenced in
arch/$ARCH/include/asm/page.h.

Mike had/has an alternate patch for Microblaze:
https://lore.kernel.org/lkml/20200630111519.GA1951986@xxxxxxxxxxxxx/

David suggested just exporting min_low_pfn & max_low_pfn in
mm/memblock.c:
https://lore.kernel.org/lkml/alpine.DEB.2.22.394.2006291911220.1118534@xxxxxxxxxxxxxxxxxxxxxxxxx/

Reported-by: kernel test robot <lkp@xxxxxxxxx>
Suggested-by: David Rientjes <rientjes@xxxxxxxxxx>
Signed-off-by: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>
Cc: linux-mm@xxxxxxxxx
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: David Rientjes <rientjes@xxxxxxxxxx>
Cc: Mike Rapoport <rppt@xxxxxxxxxxxxx>
Cc: Michal Simek <monstr@xxxxxxxxx>
Cc: Michal Simek <michal.simek@xxxxxxxxxx>
Cc: Tony Luck <tony.luck@xxxxxxxxx>
Cc: Fenghua Yu <fenghua.yu@xxxxxxxxx>
Cc: linux-ia64@xxxxxxxxxxxxxxx
---
 mm/memblock.c |    2 ++
 1 file changed, 2 insertions(+)

--- linux-next-20200825.orig/mm/memblock.c
+++ linux-next-20200825/mm/memblock.c
@@ -99,6 +99,8 @@ EXPORT_SYMBOL(contig_page_data);
 
 unsigned long max_low_pfn;
 unsigned long min_low_pfn;
+EXPORT_SYMBOL(min_low_pfn);
+EXPORT_SYMBOL(max_low_pfn);
 unsigned long max_pfn;
 unsigned long long max_possible_pfn;