selftests/powerpc: Fix compile error on tlbie_test due to newer gcc

commit 5b216ea1c40cf06eead15054c70e238c9bd4729e upstream.

Newer versions of GCC (>= 9) demand that the size of the string to be
copied must be explicitly smaller than the size of the destination.
Thus, the NULL char has to be taken into account on strncpy.

This will avoid the following compiling error:

  tlbie_test.c: In function 'main':
  tlbie_test.c:639:4: error: 'strncpy' specified bound 100 equals destination size
      strncpy(logdir, optarg, LOGDIR_NAME_SIZE);
      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  cc1: all warnings being treated as errors

Cc: stable@vger.kernel.org # v4.14
Signed-off-by: Desnes A. Nunes do Rosario <desnesn@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20191003211010.9711-1-desnesn@linux.ibm.com


[sandipan: Backported to v4.14]
Signed-off-by: Sandipan Das <sandipan@linux.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>