vxlan: fix GRO with VXLAN-GPE

In VXLAN-GPE, there may not be an Ethernet header following the VXLAN
header. But in GRO, the vxlan driver calls eth_gro_receive
unconditionally, which means the following header is incorrectly parsed
as Ethernet.

Introduce GPE specific GRO handling.

For better performance, do not check for GPE during GRO but rather
install a different set of functions at setup time.

Fixes: e1e5314d ("vxlan: implement GPE")
Reported-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Jiri Benc <jbenc@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>