odp-util: Fix VLAN parsing behavior in parse_8021q_onward().

odp-util: Fix VLAN parsing behavior in parse_8021q_onward().

Anytime there is a VLAN the flow needs to properly reflect that.  Keeping
the TPID in dl_type never makes sense and will probably cause problems.
The existing code did the right thing in the common case but not in corner
cases where it returned ODP_FIT_TOO_MUCH or ODP_FIT_TOO_LITTLE (the cases
where it returned an error don't matter since nothing looks at the flow
in that case).

Found by inspection.

Signed-off-by: Ben Pfaff <blp@nicira.com>