iptnl/ip6tnl: Unify local/remote endpoint and 6rd address parsing

iptnl/ip6tnl: Unify local/remote endpoint and 6rd address parsing

We are going to merge link_iptnl.c and link_ip6tnl.c and this is final
step to make their diffs clear and show what needs to be changed during
merge.

Note that it is safe to omit endpoint address(es) from netlink create
request as kernel is aware of such case and will use zero for that
endpoint(s).

Make sure we initialize ip6rdprefix and ip6rdrelayprefix bitlen in
link_iptnl.c only when configuring existing tunnel: if kernel does not
submit prefixlen in corresponding attributes preceeding get_addr_rta()
will set bitlen to -1 which is incorrect value.

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