tracepath, tracepath6: re-use printf return in print_host

Since the printf funcs already return the length of chars displayed,
use that value instead of re-calculating the length with strlen.

This also fixes the handling of the strlen return -- it's a size_t,
not an int.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>