datapath: compat: Fix compilation 3.11

Kernel 3.11 is only kernel where GRE APIs are available but
not vxlan. Add check for vxlan xmit to detect this case.

Reported-by: Dave Benson <dbenson@verdantnetworks.com>
Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Acked-by: Andy Zhou <azhou@nicira.com>