Improve performance in l3 submode

Improve performance in l3 submode

Two cases where performance has been improved
- When using a gateway from a routing table (l3 submode),
  store the mac within the next hop table, to avoid a hash_lookup.
  This gives ~10% improvement.
- Read tsc only once per bulk (of up to 64 packets).
  This gives ~10% improvement to swap, 4% to gen.

In addition a small fix has been added, preventig "No route" Error
to be written too aften.

Change-Id: I8a7ab74a32f09c8ff47f751ee91e84afee1b2147
Signed-off-by: Xavier Simonart <xavier.simonart@intel.com>