examples/l3fwd: remove useless reloads in FIB main loop

examples/l3fwd: remove useless reloads in FIB main loop

This patch aligns the l3fwd FIB code with the changes made to LPM in
commit 74fb854a3de6 ("examples/l3fwd: remove useless reloads in LPM
main loop").
This change ensures the compiler knows that the lcore config variables
are constant values and the compiler will then optimize the code
accordingly.

Signed-off-by: Conor Walsh <conor.walsh@intel.com>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>
Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
Reviewed-by: David Marchand <david.marchand@redhat.com>