sit: fix regression: do not release skb->dst before xmit

The sit module makes use of skb->dst in it's xmit function, so since
93f154b5

 ("net: release dst entry in dev_hard_start_xmit()") sit
tunnels are broken, because the flag IFF_XMIT_DST_RELEASE is not
unset.

This patch unsets that flag for sit devices to fix this
regression.

Signed-off-by: Sascha Hlusiak <contact@saschahlusiak.de>
Acked-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>