net: liquidio: 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 know `fw_type` must be NUL-terminated based on use here:
|       static bool fw_type_is_auto(void)
|       {
|       	return strncmp(fw_type, LIO_FW_NAME_TYPE_AUTO,
|       		       sizeof(LIO_FW_NAME_TYPE_AUTO)) == 0;
|       }
...and here
|       module_param_string(fw_type, fw_type, sizeof(fw_type), 0444);

Let's opt to NUL-pad the destination buffer as well so that we maintain
the same exact behavior that `strncpy` provided here.

A suitable replacement is `strscpy_pad` due to the fact that it
guarantees both NUL-termination and NUL-padding on the destination
buffer.

Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings [1]
Link: KSPP/linux#90
Signed-off-by: Justin Stitt <justinstitt@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20231005-strncpy-drivers-net-ethernet-cavium-liquidio-lio_main-c-v1-1-663e3f1d8f99@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: JustinStitt

drivers/net/ethernet/cavium/liquidio/lio_main.c

@@ -1689,7 +1689,7 @@ static int load_firmware(struct octeon_device *oct)
 
 	if (fw_type_is_auto()) {
 		tmp_fw_type = LIO_FW_NAME_TYPE_NIC;
-		strncpy(fw_type, tmp_fw_type, sizeof(fw_type));
+		strscpy_pad(fw_type, tmp_fw_type, sizeof(fw_type));
 	} else {
 		tmp_fw_type = fw_type;
 	}