ofproto-dpif: Fix a vlan-splinter megaflow bug

ofproto-dpif: Fix a vlan-splinter megaflow bug

When vlan-splinter is enabled, ovs receives non-vlan flows from the
kernel vlan ports, vlan tag is then added to the incoming flow before
xlating, so that they look like those received from a trunk port.

In case megaflow is enabled, xlating may set vlan masks during rule
processing as usual. If those vlan masks were serialized and downloaded
to the kernel (this bug), those mega flows will be rejected due to
unexpected vlan mask encapsulation, since the original kernel flows do
not have vlan tags. This bug does not break connectivity, but impacts
performance since all traffic received on vlan splinter ports will now
be handled by vswitchd, as no datapath flows can be successfully
installed.

This fix is to make sure no vlan mask encapsulation is generated for
the datapath flow if its in_port was re-written by vlan-splinter
receiving logic.

Bug #22567

Signed-off-by: Andy Zhou <azhou@nicira.com>