samples: pktgen: make variable consistent with option

This commit changes variable names that can cause confusion.

For example, variable DST_MIN is quite confusing since the
keyword 'udp_dst_min' and keyword 'dst_min' is used with pg_ctrl.

On the following commit, 'dst_min' will be used to set destination IP,
and the existing variable name DST_MIN should be changed.

Variable names are matched to the exact keyword used with pg_ctrl.

Signed-off-by: Daniel T. Lee <danieltimlee@gmail.com>
Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>