examples/l3fwd: skip Tx queue drain on first FIB iteration

examples/l3fwd: skip Tx queue drain on first FIB iteration

The commit a8f8b672d575 ("examples/l3fwd: skip Tx queue drain on first iteration")
implemented a change to the EM and LPM lookup methods to
prevent the Tx queue drain running in the first iteration of their
packet processing loops.

This patch applies the same change into the new FIB lookup method.

Signed-off-by: Conor Walsh <conor.walsh@intel.com>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>
Acked-by: Vladimir Medvedkin <vladimir.medvedkin@intel.com>