ip6/tunnel: Fix tclass output

ip6/tunnel: Fix tclass output

In link_gre6.c it seems copy paste error: tclass is 8 bits,
not 20 as flowlabel.

In link_iptnl.c rename "flowinfo_tclass" to "tclass" as it
correct name since flowinfo is implementation internal name
used to label combined within u32 attribute tclass and
flowlabel.

Fixes: 1facc1c61c07 ("ip: link_ip6tnl.c: add json output support")
Fixes: 2e706e12d9b0 ("Merge branch 'master' into net-next")
Signed-off-by: Serhey Popovych <serhe.popovych@gmail.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>