[PATCH] ieee80211: Add helpers for IBSS DFS handling

To support IEEE 802.11h in IBSS, an ibss_dfs field is added to struct
ieee80211_network. In IBSS, if one STA sends a beacon with DFS info
(for radar detection), all the other STAs should receive and store
this DFS.  All STAs should send the DFS as one of the information
element in the beacon they are scheduled to send (if possible) in
the future.

Since the ibss_dfs has variable length, it must be allocated
dynamically. ieee80211_network_reset() is added to clear the ibss_dfs
field. ieee80211_network_free() is also updated to free the ibss_dfs
field if it is not NULL.

Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>