ping: Make in_cksum() operate on u_char buffer

This fixes -Wcast-align errors for in_cksum() calls when compiled with
WARNS=6.

Submitted by: Ján Sučan <sucanjan@gmail.com>
MFC after: 2 weeks
Sponsored by: Google, inc. (Google Summer of Code 2019)
Differential Revision: https://reviews.freebsd.org/D21261