wifi: airo: 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.

`extra` is clearly supposed to be NUL-terminated which is evident by the
manual NUL-byte assignment as well as its immediate usage with strlen().

Moreover, let's NUL-pad since there is deliberate effort (48 instances)
made elsewhere to zero-out buffers in these getters and setters:
6050 | memset(local->config.nodeName, 0, sizeof(local->config.nodeName));
6130 | memset(local->config.rates, 0, 8);
6139 | memset(local->config.rates, 0, 8);
6414 | memset(key.key, 0, MAX_KEY_SIZE);
6497 | memset(extra, 0, 16);
(to be clear, strncpy also NUL-padded -- we are matching that behavior)

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

We can also replace the hard-coded size of "16" to IW_ESSID_MAX_SIZE
because this function is a wext handler.

In wext-core.c we have:
static const struct iw_ioctl_description standard_ioctl[] = {
...
        [IW_IOCTL_IDX(SIOCGIWNICKN)] = {
                .header_type    = IW_HEADER_TYPE_POINT,
                .token_size     = 1,
                .max_tokens     = IW_ESSID_MAX_SIZE,
        },

So the buffer size is (strangely) IW_ESSID_MAX_SIZE

Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings [1]
Link: https://github.com/KSPP/linux/issues/90
Cc: linux-hardening@vger.kernel.org
Signed-off-by: Justin Stitt <justinstitt@google.com>
Reviewed-by: Jeff Johnson <quic_jjohnson@quicinc.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20231026-strncpy-drivers-net-wireless-cisco-airo-c-v2-1-413427249e47@google.com

drivers/net/wireless/cisco/airo.c

@@ -6067,8 +6067,7 @@ static int airo_get_nick(struct net_device *dev,
 	struct airo_info *local = dev->ml_priv;
 
 	readConfigRid(local, 1);
-	strncpy(extra, local->config.nodeName, 16);
-	extra[16] = '\0';
+	strscpy_pad(extra, local->config.nodeName, IW_ESSID_MAX_SIZE);
 	dwrq->length = strlen(extra);
 
 	return 0;