mac80211: free skb fraglist before freeing the skb

mac80211 uses the frag list to build AMSDU. When freeing
the skb, it may not be really freed, since someone is still
holding a reference to it.
In that case, when TCP skb is being retransmitted, the
pointer to the frag list is being reused, while the data
in there is no longer valid.
Since we will never get frag list from the network stack,
as mac80211 doesn't advertise the capability, we can safely
free and nullify it before releasing the SKB.

Signed-off-by: Sara Sharon <sara.sharon@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>