fbdev: fsl-diu-fb: replace deprecated strncpy with strscpy_pad

strncpy() is deprecated for use on NUL-terminated destination strings
[1] and as such we should prefer more robust and less ambiguous string
interfaces.

A better alternative is strscpy() as it guarantees NUL-termination on
the destination buffer.

Since we are eventually copying over to userspace, let's ensure we
NUL-pad the destination buffer by using the pad variant of strscpy.
- core/fb_chrdev.c:
234 |	err = copy_to_user(&fix32->id, &fix->id, sizeof(fix32->id));

Furthermore, we can use the new 2-argument variants of strscpy() and
strscpy_pad() introduced by Commit e6584c39

 ("string: Allow
2-argument strscpy()") to simplify the syntax even more.

Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings [1]
Link: https://manpages.debian.org/testing/linux-manual-4.8/strscpy.9.en.html [2]
Link: https://github.com/KSPP/linux/issues/90
Cc: linux-hardening@vger.kernel.org
Signed-off-by: Justin Stitt <justinstitt@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Helge Deller <deller@gmx.de>