[NET]: Fix MAX_HEADER setting.

MAX_HEADER is either set to LL_MAX_HEADER or LL_MAX_HEADER + 48, and
this is controlled by a set of CONFIG_* ifdef tests.

It is trying to use LL_MAX_HEADER + 48 when any of the tunnels are
enabled which set hard_header_len like this:

dev->hard_header_len = LL_MAX_HEADER + sizeof(struct xxx);

The correct set of tunnel drivers which do this are:

ipip
ip_gre
ip6_tunnel
sit

so make the ifdef test match.

Noticed by Patrick McHardy and with help from Herbert Xu.

Signed-off-by: David S. Miller <davem@davemloft.net>