wifi: mac80211: check skb_shared in ieee80211_8023_xmit()

Add a missing skb_shared check into 802.3 path to prevent potential
use-after-free from happening. This also uses skb_share_check()
instead of open-coding in tx path.

Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Link: https://lore.kernel.org/r/e7a73aaf7742b17e43421c56625646dfc5c4d2cb.1653571902.git.ryder.lee@mediatek.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>