Improve l3fwd performance

Improve l3fwd performance

JIRA: SAMPLEVNF-128

Improve performance of l3fwd use-case.
L3fwd did not disable TX offload flags, which resulted in
non-vector mode to be used in pmd in some cases like bare-metal.
Note that in some cases such as virtio, vector mode was already
used as OVS reports that it does not support TX-offload, hence
TX-offload was already disabled on virtio.
Using vector mode usually brings higher performance.

Change-Id: I102b846d604a9cf28c6b7dae8bedbe918ccfcf6c
Signed-off-by: Xavier Simonart <xavier.simonart@intel.com>