tipc: make sure IPv6 header fits in skb headroom

commit 9bd160bf upstream.

Expand headroom further in order to be able to fit the larger IPv6
header. Prior to this patch this caused a skb under panic for certain
tipc packets when using IPv6 UDP bearer(s).

Signed-off-by: Richard Alpe <richard.alpe@ericsson.com>
Acked-by: Jon Maloy <jon.maloy@ericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Jon Maloy <jon.maloy@ericsson.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>