examples/l3fwd: remove useless reloads in EM main loop

examples/l3fwd: remove useless reloads in EM main loop

This patch aligns the l3fwd EM 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>
Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
Reviewed-by: David Marchand <david.marchand@redhat.com>