nl80211: Add NL80211_FLAG_CLEAR_SKB flag for other NL commands

This commit adds NL80211_FLAG_CLEAR_SKB flag to other NL commands
that carry key data to ensure they do not stick around on heap
after the SKB is freed.

Also introduced this flag for NL80211_CMD_VENDOR as there are sub
commands which configure the keys.

Signed-off-by: Sunil Dutt <usdutt@codeaurora.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>