ofproto/trace: Use final flow to compute "Relevant fields".

Commit bcd2633a (ofproto-dpif: Store relevant fields for wildcarding
in facet.) introduced the bug that uses original input flow as final
flow to compute the "Relevant fields" in ofproto/trace ouput.  This
commit fixes this bug.

Signed-off-by: Alex Wang <alexw@nicira.com>
ofproto/trace: Remove the unused variables.

This commit removes the unused variables in ofproto_trace() function.

Signed-off-by: Alex Wang <alexw@nicira.com>
ofproto/trace: Change output field name.

This commit changes the output field name from "Relevant fields"
to "Megaflow".

Signed-off-by: Alex Wang <alexw@nicira.com>
ofproto/trace: Show megaflow fields in each resubmit.

This commit makes the ofproto/trace show the megaflow fields
for each resubmit.

Signed-off-by: Alex Wang <alexw@nicira.com>