vti4: Don't count header length twice.

We currently count the size of LL_MAX_HEADER and struct iphdr
twice for vti4 devices, this leads to a wrong device mtu.
The size of LL_MAX_HEADER and struct iphdr is already counted in
ip_tunnel_bind_dev(), so don't do it again in vti_tunnel_init().

Fixes: b9959fd3 ("vti: switch to new ip tunnel code")
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>