batman-adv: Check type of x and y in seq_(before|after)

seq_before and seq_after depend on the fact that both sequence numbers
have the same type and thus the same bitwidth. We can ensure that by
compile time checking using a compare between the pointer to the
temporary buffers which were created using the typeof of both
parameters. For example gcc would create a warning like
"warning: comparison of distinct pointer types lacks a cast".

Signed-off-by: Sven Eckelmann <sven@narfation.org>