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

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