fbdev: uvesafb: 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.

We expect v86d_path to be NUL-terminated based on its use with the
C-string format specifier in printf-likes:
|	pr_err("failed to execute %s\n", v86d_path);
and
|	return snprintf(buf, PAGE_SIZE, "%s\n", v86d_path);

Let's also opt to pad v86d_path since it may get used in and around
userspace:
|	return call_usermodehelper(v86d_path, argv, envp, UMH_WAIT_PROC);

Considering the above, strscpy_pad() is the best replacement as it
guarantees both NUL-termination and NUL-padding on the destination
buffer.

Note that this patch relies on the _new_ 2-argument versions of
strscpy() and strscpy_pad() introduced in Commit e6584c3 ("string:
Allow 2-argument strscpy()").

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
Link: KSPP#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>

Author: JustinStitt

drivers/video/fbdev/uvesafb.c

@@ -1867,7 +1867,7 @@ static ssize_t v86d_show(struct device_driver *dev, char *buf)
 static ssize_t v86d_store(struct device_driver *dev, const char *buf,
 		size_t count)
 {
-	strncpy(v86d_path, buf, PATH_MAX - 1);
+	strscpy_pad(v86d_path, buf);
 	return count;
 }
 static DRIVER_ATTR_RW(v86d);