mac80211: fix potentially redundant skb data copying

When an skb is shared, it needs to be duplicated, along with its data buffer.
If the skb does not have enough headroom, using skb_copy might cause the data
buffer to be copied twice (once by skb_copy and once by pskb_expand_head).
Fix this by using skb_clone initially and letting ieee80211_skb_resize sort
out the rest.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>