datapath: Re-designate OVS_FRAGMENT_BACKPORT.

datapath: Re-designate OVS_FRAGMENT_BACKPORT.

Typically the way that we include backported code is by testing for
existence of the feature in the upstream codebase via header checks,
then attempt to use the upstream code as much as possible. However, for
the IP fragmentation handling backport we have an additional constraint
which is that we cannot support kernels older than Linux-3.10.

To date, OVS_FRAGMENT_BACKPORT has been defined to include the backport
of the IP fragmentation code for all kernels from 3.10 to 4.2, rather
than attempting to use the upstream code as much as possible. This patch
relaxes OVS_FRAGMENT_BACKPORT to only check the lower bound so that the
upstream code may be used in more circumstances.

Signed-off-by: Joe Stringer <joe@ovn.org>
Acked-by: Pravin B Shelar <pshelar@ovn.org>