examples/l3fwd: fix size of destination port ids

examples/l3fwd: fix size of destination port ids

Originally l3fwd used 16-bit value to store dest_port value.
To accommodate 24-bit nexthop dest_port was increased to 32-bit,
though some further packet processing code remained unchanged and
still expects dest_port to be 16-bit.
That is not correct and can cause l3fwd invalid behaviour or even
process crash/hang on some input packet patterns.
For the fix, I choose the simplest approach and restored dest_port
as 16-bit value, plus necessary conversions from 32 to 16 bit values
after lpm_lookupx4.

Fixes: dc81ebbacaeb ("lpm: extend IPv4 next hop field")

Signed-off-by: Konstantin Ananyev <konstantin.ananyev@intel.com>