[NET]: Avoid copying writable clones in tunnel drivers

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --cc net/ipv4/ip_gre.c
Simple merge

diff --cc net/ipv4/ipip.c
Simple merge

diff --cc net/ipv6/ip6_tunnel.c
Simple merge

diff --cc net/ipv6/sit.c
Simple merge