openvswitch: Move LRO check from transmit to receive.

The check for LRO packets was incorrectly put in the transmit path
instead of on receive.  Since this check is supposed to protect OVS
(and other parts of the system) from packets that it cannot handle
it is obviously not useful on egress.  Therefore, this commit moves
it back to the receive side.

The primary problem that this caused is upcalls to userspace tried
to segment the packet even though no segmentation information is
available.  This would later cause NULL pointer dereferences when
skb_gso_segment() did nothing.

Signed-off-by: Jesse Gross <jesse@nicira.com>