mac80211_hwsim: fix error handling in tx_frame_nl

Correct two problems with the error handling when using the netlink
forwarding API: first, the netlink skb is never freed if nla_put()
fails; and second, genlmsg_unicast() can fail if the netlink socket
is full.  In the latter case, the corresponding data skb is not counted
as a drop and userspace programs like wmediumd will see TCP stalls
due to lost packets.

Signed-off-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>