arch/powerpc/kernel: Use %12.12s instead of %12s to avoid memory overflow

for tmp_part->header.name:
    it is "Terminating null required only for names < 12 chars".
    so need to limit the %.12s for it in printk

  additional info:

    %12s  limit the width, not for the original string output length
          if name length is more than 12, it still can be fully displayed.
          if name length is less than 12, the ' ' will be filled before name.

    %.12s truly limit the original string output length (precision)

Signed-off-by: Chen Gang <gang.chen@asianux.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>