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

module/os/freebsd/spl/spl_kstat.c

@@ -231,6 +231,7 @@ restart:
 	}
 	free(ksp->ks_raw_buf, M_TEMP);
 	mutex_exit(ksp->ks_lock);
+	sbuf_trim(sb);
 	rc = sbuf_finish(sb);
 	if (rc == 0)
 		rc = SYSCTL_OUT(req, sbuf_data(sb), sbuf_len(sb));