radv: replace memset()+strcpy() with snprintf()

Just like the next line :)

Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
13 jobs for !1289 with radv-sprintf in 9 minutes and 14 seconds (queued for 2 seconds)
merge request