batman-adv: Remove hdr_size skb size check in batadv_interface_rx

The callers of batadv_interface_rx have to make sure that enough data can
be pulled from the skb when they read the batman-adv header. The only two
functions using it are either calling pskb_may_pull with hdr_size directly
(batadv_recv_bcast_packet) or indirectly via batadv_check_unicast_packet
(batadv_recv_unicast_packet).

Reported-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Antonio Quartulli <a@unstable.cc>