batman-adv: Add lower layer needed_(head|tail)room to own ones

The maximum of hard_header_len and maximum of all needed_(head|tail)room of
all slave interfaces of a batman-adv device must be used to define the
batman-adv device needed_(head|tail)room. This is required to avoid too
small buffer problems when these slave devices try to send the encapsulated
packet in a tx path without the possibility to resize the skbuff.

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Antonio Quartulli <antonio@meshcoding.com>