compilation error on centos 7 kernels

[Yousong Zhou]

Yesterday I tried compiling openvswitch-dkms on 2 centos 7 systems,
both failed out of box because of the compat conditional compilation
code

Both system has LINUX_VERSION_CODE 199168, i.e. 3.10.0.  The 1st runs
kernel 3.10.0-514.26.2.el7.x86_64, and the second runs
3.10.0-693.2.2.el7.x86_64.  They both lacks skb_reset_tc,
our_pskb_put(), genl_family_attrbuf(), skb_put_data().  And the
3.10.0-514 kernel also lacks ipv6_mod_enabled() so I had to prepare
different dkms patches for them ;( . The bright side is that currently
both systems work fine ;)

It's partially caused by commit 401453 ("compat: handle RHEL 7.5's
recent backports"), but I think the code will be very hard to read and
maintain if we continue to use the current macro detection logic.  I
remembered openvswitch linux kernel datapath module was already in
such an state.  Maybe compile test will do better.

                yousong