mac80211: ensure that mgmt tx skbs have tailroom for encryption

mac80211: ensure that mgmt tx skbs have tailroom for encryption

commit 9d0f50b80222dc273e67e4e14410fcfa4130a90c upstream.

Some drivers use IEEE80211_KEY_FLAG_SW_MGMT_TX to indicate that management
frames need to be software encrypted. Since normal data packets are still
encrypted by the hardware, crypto_tx_tailroom_needed_cnt gets decremented
after key upload to hw. This can lead to passing skbs to ccmp_encrypt_skb,
which don't have the necessary tailroom for software encryption.

Change the code to add tailroom for encrypted management packets, even if
crypto_tx_tailroom_needed_cnt is 0.

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
[bwh: Backported to 3.16: we always expand cloned skbs here; don't change that]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>