mac80211: verify that skb data is present

commit 9b395bc3be1cebf0144a127c7e67d56dbdac0930 upstream.

A number of places in the mesh code don't check that
the frame data is present and in the skb header when
trying to access. Add those checks and the necessary
pskb_may_pull() calls. This prevents accessing data
that doesn't actually exist.

To do this, export ieee80211_get_mesh_hdrlen() to be
able to use it in mac80211.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>