Make dbufstat work on FreeBSD

With procfs_list kstats implemented for FreeBSD, dbufs are now exposed
as kstat.zfs.misc.dbufs.

On FreeBSD, dbufstats can use the sysctl instead of procfs when no
input file has been given.

Enable the dbufstats tests on FreeBSD.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <freqlabs@FreeBSD.org>
Closes #11008

tests/zfs-tests/include/libtest.shlib

@@ -4154,18 +4154,36 @@ function ls_xattr # path
 	esac
 }
 
+function kstat # stat flags?
+{
+	typeset stat=$1
+	typeset flags=${2-"-n"}
+
+	case $(uname) in
+	FreeBSD)
+		sysctl $flags kstat.zfs.misc.$stat
+		;;
+	Linux)
+		typeset zfs_kstat="/proc/spl/kstat/zfs/$stat"
+		[[ -f "$zfs_kstat" ]] || return 1
+		cat $zfs_kstat
+		;;
+	*)
+		false
+		;;
+	esac
+}
+
 function get_arcstat # stat
 {
 	typeset stat=$1
 
 	case $(uname) in
 	FreeBSD)
-		sysctl -n kstat.zfs.misc.arcstats.$stat
+		kstat arcstats.$stat
 		;;
 	Linux)
-		typeset zfs_arcstats="/proc/spl/kstat/zfs/arcstats"
-		[[ -f "$zfs_arcstats" ]] || return 1
-		grep $stat $zfs_arcstats | awk '{print $3}'
+		kstat arcstats | awk "/$stat/ { print \$3 }"
 		;;
 	*)
 		false