compat: Detect and use upstream ip_fragment().

compat: Detect and use upstream ip_fragment().

Previously a version check was used to determine whether the upstream
ip_fragment() should be used or the backported version. The actual test
is for whether upstream commit d6b915e29f4a ("ip_fragment: don't forward
defragmented DF packet") is present, so test for that instead.

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