gre6/tunnel: Do not submit garbage in flowinfo

gre6/tunnel: Do not submit garbage in flowinfo

We always send flowinfo to the kernel. If flowlabel/tclass
was set first to non-inherit value and then reset to
inherit we do not clear flowlabel/tclass part in flowinfo,
send it to kernel and can get from the kernel back.

Even if we check for IP6_TNL_F_USE_ORIG_TCLASS and
IP6_TNL_F_USE_ORIG_FLOWLABEL when printing options
sending invalid flowlabel/tclass to the kernel seems
bad idea.

Note that ip6tnl always clean corresponding flowinfo
parts on inherit.

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