ping6: Remove unnecessary level of indirection from dnsdecode() parameter

The `sp' pointer doesn't need to be modified in the caller of
dnsdecode().

This fixes -Wcast-qual error (`must have all intermediate pointers
const qualified to be safe') 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/D21215