mac80211: fix the skb cloned check in the tx path

Using skb_header_cloned to check if it's safe to write to the skb is not
enough - mac80211 also touches the tailroom of the skb.
Initially this check was only used to increase a counter, however this
commit changed the code to also skip skb data reallocation if no extra
head/tailroom was needed:

commit 4cd06a34
mac80211: skip unnecessary pskb_expand_head calls

It added a regression at least with iwl3945, which is fixed by this patch.

Reported-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Tested-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>