sit/rtnl: add missing parameters on dump

IFLA_IPTUN_FLAGS and IFLA_IPTUN_PMTUDISC were missing.
There is only one possible flag in i_flag: SIT_ISATAP.

Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>