wifi: mwifiex: prevent authentication frame length truncation

In the Linux kernel, the following vulnerability has been resolved: wifi: mwifiex: prevent authentication frame length truncation mwifiex_cfg80211_authenticate() derives the authentication frame length from req->ie_len and req->auth_data_len, both of type size_t, but stores it in a u16. NL80211_ATTR_AUTH_DATA only has a minimum length policy. Since nla_len is a u16, a single attribute can carry up to 65531 bytes of payload, so the sum can exceed U16_MAX before it is assigned to pkt_len. The truncated pkt_len determines the skb frame area, while the copy length remains req->auth_data_len - 4, resulting in a heap buffer overflow. For example, with auth_data_len equal to 65510 and no IEs, the sum is 65546. It is truncated to 10 and then reduced by four to 6. The driver appends only six bytes to the skb with skb_put(), but then copies 65506 user-provided bytes into the authentication body. Reaching this path requires CAP_NET_ADMIN in the user namespace owning the network namespace, an up station netdev, and a suitable BSS/SAE authentication request. Compute the length in size_t, reject values that cannot be represented by the firmware's u16 frame length field, and only then assign it to pkt_len.