Move spa_stats.c to common code

Initially it was considered simplest to stub out all
of the functions on FreeBSD. Now that FreeBSD supports
KSTAT_TYPE_RAW at least some of the functionality should
be made available.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Richard Elling <Richard.Elling@RichardElling.com>
Signed-off-by: Matt Macy <mmacy@FreeBSD.org>
Closes #10842

module/zfs/Makefile.in

@@ -76,6 +76,7 @@ $(MODULE)-objs += spa_errlog.o
 $(MODULE)-objs += spa_history.o
 $(MODULE)-objs += spa_log_spacemap.o
 $(MODULE)-objs += spa_misc.o
+$(MODULE)-objs += spa_stats.o
 $(MODULE)-objs += space_map.o
 $(MODULE)-objs += space_reftree.o
 $(MODULE)-objs += txg.o