netdev-dpdk: Refactor netdev_dpdk structure.

netdev-dpdk: Refactor netdev_dpdk structure.

This commit introduces below changes to netdev_dpdk structure.

- Mark cachelines and reorder few member variables.
- Maintain the grouping of related member variables.
- Add comment on the information on pad bytes where ever appropriate, so
  new members can be introduced in the future to fill the gaps.

  Below is how this structure looks with this commit.

                  Member                    size

         OVS_CACHE_LINE_MARKER cacheline0;
             dpdk_port_t port_id;            1
             bool attached;                  1
             ...

         OVS_CACHE_LINE_MARKER cacheline1;
             struct ovs_mutex;              48
             struct dpdk_mp *dpdk_mp;        8
             ...

Signed-off-by: Bhanuprakash Bodireddy <bhanuprakash.bodireddy@intel.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>