mac80211: Build TX radiotap header dynamically

Get rid of the ieee80211_tx_status_rtap_hdr struct and instead build the
rtap header dynamically. This makes it easier to extend the rtap header
generation in the future.

Add ieee80211_tx_radiotap_len to calculate the expected size of the
rtap header before generating it. Since we can't check if the rtap
header fits into the requested headroom during compile time anymore
add a WARN_ON_ONCE.

Also move the actual rtap header generation into its own function.

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>