batman-adv: create common header for ICMP packets

the icmp and the icmp_rr packets share the same initial
fields since they use the same code to be processed and
forwarded.

Extract the common fields and put them into a separate
struct so that future ICMP packets can be easily added
without bloating the packet definition.

However, keep the seqno field outside of the newly created
common header because future ICMP types may require a
bigger sequence number space.

This change breaks compatibility due to fields reordering
in the ICMP headers.

Signed-off-by: Antonio Quartulli <antonio@open-mesh.com>
Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>