Cast pointer to uintptr_t to avoid alignment warnings.

Both struct ip and struct udphdr both have an aligment of 2, but the
cast from struct ip to a uint32_t pointer confused GCC 9 into raising
the required alignment to 4 and then raising a
-Waddress-of-packed-member error when casting to struct udphdr.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D31941