datapath: Fix tunnel source port selection for mega flow

datapath: Fix tunnel source port selection for mega flow

Tunnel source port selection was based on hash value cached in the
flow. This no longer works with mega flow, since all flows matching
a mega flow will be transmitted with the same tunnel source port.

This patch computes the tunnel source port at run time based on each
incoming packet. Packets belong to the same micro flow would still get
the same source port, but multiple micro flows hitting the same mega flow
can get different source ports.

Packets injected from the usespace will be assigned to the same
source port as if they are forwarded in the kernel.

Bug #18216

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