batman-adv: Fix incorrect offset in batadv_tt_tvlv_ogm_handler_v1()

Since commit 4436df47 ("batman-adv: Add flex array to struct
batadv_tvlv_tt_data"), the introduction of batadv_tvlv_tt_data's flex
array member in batadv_tt_tvlv_ogm_handler_v1() put tt_changes at
invalid offset. Those TT changes are supposed to be filled from the end
of batadv_tvlv_tt_data structure (including vlan_data flexible array),
but only the flex array size is taken into account missing completely
the size of the fixed part of the structure itself.

Fix the tt_change offset computation by using struct_size() instead of
flex_array_size() so both flex array member and its container structure
sizes are taken into account.

Cc: stable@vger.kernel.org
Fixes: 4436df47 ("batman-adv: Add flex array to struct batadv_tvlv_tt_data")
Signed-off-by: Remi Pommarel <repk@triplefau.lt>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>