batman-adv: fix size of batadv_icmp_header

struct batadv_icmp_header currently has a size of 17, which
will be padded to 20 on some architectures. Fix this by
unrolling the header into the parent structures.

Moreover keep the ICMP parsing functions as generic as they
are now by using a stub icmp_header struct during packet
parsing.

Signed-off-by: Antonio Quartulli <antonio@meshcoding.com>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>