docs: networking: Replace strncpy() with strscpy()

Replace example code's use of strncpy() with strscpy() functions. Using
strncpy() is considered deprecated:
https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings



Signed-off-by: Kees Cook <keescook@chromium.org>
Acked-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Link: https://lore.kernel.org/r/20210602202914.4079123-1-keescook@chromium.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>