datapath: compat: Fix build on RHEL 7.5

datapath: compat: Fix build on RHEL 7.5

1) OVS datapath compat modules breaks on RHEL 7.5, because it moves
ndo_change_mtu function pointer from 'struct net_device_ops' to
'struct net_device_ops_extended'.

2) RHEL 7.5 introduces the MTU range checking as mentioned in
6c0bf091 ("datapath: use core MTU range checking in core net infra").
However, the max_mtu field is defined in 'struct net_device_extended'
but not in 'struct net_device' as upstream kernel.

This patch defines a new symbol HAVE_RHEL7_MAX_MTU that determines
the previous 2 conditions, and fixes the backport issue.

Signed-off-by: Yi-Hung Wei <yihung.wei@gmail.com>
Acked-by: Lucas Alvares Gomes <lucasagomes@gmail.com>
Signed-off-by: Pravin B Shelar <pshelar@ovn.org>