ip/tunnel: Unify local/remote endpoint address printing

ip/tunnel: Unify local/remote endpoint address printing

Introduce and use tnl_print_endpoint() helper to print of tunnel
endpoint address.

Note that for AF_INET and AF_INET6 inet_ntop(3) is used that may return
NULL in case of failure and while unlikely format_host_rta() might
return NULL too. Handle this case when passing local/remote to
print_string().

Signed-off-by: Serhey Popovych <serhe.popovych@gmail.com>
Signed-off-by: David Ahern <dsahern@gmail.com>