datapath: Calculate flow hash after extracting metadata.

datapath: Calculate flow hash after extracting metadata.

When we execute a packet from userspace we first extract the header
fields from the packet and then add supplied metadata.  However, we
compute the hash of the packet in between these two steps despite
the fact that the metadata can affect the hash.  This can lead to
two separate hashes for packets of the same flow.

Found by code inspection, not an actual real-world problem.

Signed-off-by: Jesse Gross <jesse@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>