vxlan: GRO support at tunnel layer

vxlan: GRO support at tunnel layer

Add calls to gro_cells infrastructure to do GRO when receiving on a tunnel.

Testing:

Ran 200 netperf TCP_STREAM instance

  - With fix (GRO enabled on VXLAN interface)

    Verify GRO is happening.

    9084 MBps tput
    3.44% CPU utilization

  - Without fix (GRO disabled on VXLAN interface)

    Verified no GRO is happening.

    9084 MBps tput
    5.54% CPU utilization

Signed-off-by: Tom Herbert <tom@herbertland.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Orabug: 25879842
(cherry picked from commit 58ce31cca1ffe057f4744c3f671e3e84606d3d4a)
Signed-off-by: Cathy Zhou <Cathy.Zhou@Oracle.COM>
Reviewed-by: Jack Vogel <jack.vogel@oracle.com>