batman-adv: fix wrong dhcp option list browsing

In is_type_dhcprequest(), while parsing a DHCP message, if the entry we found in
the option list is neither a padding nor the dhcp-type, we have to ignore it and
jump as many bytes as its length + 1. The "+ 1" byte is given by the subtype
field itself that has to be jumped too.

Reported-by: Marek Lindner <lindner_marek@yahoo.de>
Signed-off-by: Antonio Quartulli <ordex@autistici.org>