wifi: rsi: validate beacon length before fixed buffer copy

wifi: rsi: validate beacon length before fixed buffer copy

rsi_prepare_beacon() copies the mac80211 beacon frame after
FRAME_DESC_SZ into a management skb whose usable tailroom may be smaller
than MAX_MGMT_PKT_SIZE after alignment.

Validate the beacon length against the actual tailroom before the copy
and skb_put(). Leave ownership of the management skb with the caller on
error, matching the existing rsi_send_beacon() cleanup path.

Signed-off-by: Pengpeng Hou <pengpeng@iscas.ac.cn>
Link: https://patch.msgid.link/20260705084824.68105-1-pengpeng@iscas.ac.cn
Signed-off-by: Johannes Berg <johannes.berg@intel.com>