Shorten sizeof to u_int, to match the %u format used with it.

On LP64 and LLP64 platforms, sizeof returns a 64-bit value, which is
larger than an int or unsigned int, so if you add the result of sizeof
to a value shorter than 64 bits, you can't print the result with %u.

As an M3UA parameter header is much shorter than 2^32 bytes, we can
safely just cast sizeof(struct m3ua_param_header) to u_int.