mm: fix RODATA_TEST failure "rodata_test: test data was not read only"

On powerpc, RODATA_TEST fails with message the following messages:

  Freeing unused kernel memory: 528K
  rodata_test: test data was not read only

This is because GCC allocates it to .data section:

  c0695034 g     O .data	00000004 rodata_test_data

Since commit 056b9d8a7692 ("mm: remove rodata_test_data export, add
pr_fmt"), rodata_test_data is used only inside rodata_test.c By
declaring it static, it gets properly allocated into .rodata section
instead of .data:

  c04df710 l     O .rodata	00000004 rodata_test_data

Fixes: 056b9d8a7692 ("mm: remove rodata_test_data export, add pr_fmt")
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Christophe Leroy <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Jinbum Park <[email protected]>
Cc: Segher Boessenkool <[email protected]>
Cc: David Laight <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
diff --git a/mm/rodata_test.c b/mm/rodata_test.c
index 6bb4deb..d908c87 100644
--- a/mm/rodata_test.c
+++ b/mm/rodata_test.c
@@ -14,7 +14,7 @@
 #include <linux/uaccess.h>
 #include <asm/sections.h>
 
-const int rodata_test_data = 0xC3;
+static const int rodata_test_data = 0xC3;
 
 void rodata_test(void)
 {