Make IP statistics counters per-CPU so they can be updated safely.

Make IP statistics counters per-CPU so they can be updated safely.

	* For SMP, we index into the array with the current CPU's
	  as ipstats_ary[gd->gd_cpuid], and for UP we just return
	  struct at ipstats_ary[0] (CPU-0).

	* Rename the structure from ipstat to ip_stats.

	* Wrap function definition of CPU counter aggregation in a
	  macro called CPU_STATS_FUNC() to avoid duplication.

	* Retain support for resetting the counters using netstat(1).