gre: clear IFF_TX_SKB_SHARING

ether_setup sets IFF_TX_SKB_SHARING but this is not supported by gre
as it modifies the skb on xmit.

Also, clean up whitespace in ipgre_tap_setup when we're already touching it.

Signed-off-by: Jiri Benc <jbenc@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>