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 056b9d8a ("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: 056b9d8a ("mm: remove rodata_test_data export, add pr_fmt")
Link: http://lkml.kernel.org/r/20170921093729.1080368AC1@po15668-vm-win7.idsi0.si.c-s.fr


Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Cc: Kees Cook <keescook@chromium.org>
Cc: Jinbum Park <jinb.park7@gmail.com>
Cc: Segher Boessenkool <segher@kernel.crashing.org>
Cc: David Laight <David.Laight@ACULAB.COM>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>