wifi: mac80211: drop oversized fragments to avoid extra_len overflow

In the Linux kernel, the following vulnerability has been resolved:

wifi: mac80211: drop oversized fragments to avoid extra_len overflow

In ieee80211_rx_h_defragment(), fragment payloads are accumulated into
entry->extra_len as subsequent fragments arrive:

entry->extra_len += rx->skb->len;

When the final fragment arrives, the head fragment is dequeued, and
pskb_expand_head() is called with entry->extra_len to ensure sufficient
tailroom for all queued fragments before copying:

if (skb_tailroom(rx->skb) < entry->extra_len) {
if (unlikely(pskb_expand_head(rx->skb, 0, entry->extra_len,
GFP_ATOMIC))) {
...
}
}
while ((skb = __skb_dequeue(&entry->skb_list))) {
skb_put_data(rx->skb, skb->data, skb->len);
dev_kfree_skb(skb);
}

In commit 69f132236827 ("mac80211: shrink struct
ieee80211_fragment_entry"), entry->extra_len was narrowed from unsigned
int to u16 in order to reduce structure memory footprint.

However, an IEEE 802.11 frame sequence can contain up to 16 fragments
(frag numbers 0..15). If a sequence of large fragments arrives (e.g. from
a malicious peer or rogue AP), the sum of fragment lengths can exceed
65535 bytes (for example, 15 fragments of 4400 bytes total 66000 bytes).
Because extra_len is a u16, this addition overflows and wraps around
modulo 65536 (e.g. 66000 wraps to 464).

Consequently, pskb_expand_head() allocates only the truncated amount of
tailroom (or is skipped entirely if the head fragment already has >= 464
bytes of tailroom). When skb_put_data() subsequently iterates through the
queued skb list, it appends the full payload into the undersized buffer,
causing skb_put() to trigger skb_over_panic() and crash the kernel in
softirq context.

A legitimate MSDU in IEEE 802.11 is at most 2304 bytes (or up to 7935/11454
bytes for A-MSDU, which is not fragmented), well below U16_MAX.

Fix this without increasing the size of struct ieee80211_fragment_entry by
checking whether adding the incoming fragment length would exceed U16_MAX.
If so, purge the queued fragments and drop the frame.

[assign a separate drop reason]