FreeBSD: Skip RAW kstat sysctls by default

These kstats are often expensive to compute so we want to avoid them
unless specifically requested.

The following kstats are affected by this change:

kstat.zfs.${pool}.multihost
kstat.zfs.${pool}.misc.state
kstat.zfs.${pool}.txgs
kstat.zfs.misc.fletcher_4_bench
kstat.zfs.misc.vdev_raidz_bench
kstat.zfs.misc.dbufs
kstat.zfs.misc.dbgmsg

In FreeBSD 13, sysctl(8) has been updated to still list the
names/description/type of skipped sysctls so they are still
discoverable.

Reviewed-by: Allan Jude <allan@klarasystems.com>
Reviewed-by: Mateusz Guzik <mjguzik@gmail.com>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes openzfs#11099

module/os/freebsd/spl/spl_kstat.c

@@ -475,14 +475,14 @@ kstat_install(kstat_t *ksp)
 		if (ksp->ks_raw_ops.data) {
 			root = SYSCTL_ADD_PROC(&ksp->ks_sysctl_ctx,
 			    SYSCTL_CHILDREN(ksp->ks_sysctl_root),
-			    OID_AUTO, ksp->ks_name,
-			    CTLTYPE_STRING | CTLFLAG_RD | CTLFLAG_MPSAFE,
+			    OID_AUTO, ksp->ks_name, CTLTYPE_STRING | CTLFLAG_RD
+			    | CTLFLAG_MPSAFE | CTLFLAG_SKIP,
 			    ksp, 0, kstat_sysctl_raw, "A", ksp->ks_name);
 		} else {
 			root = SYSCTL_ADD_PROC(&ksp->ks_sysctl_ctx,
 			    SYSCTL_CHILDREN(ksp->ks_sysctl_root),
-			    OID_AUTO, ksp->ks_name,
-			    CTLTYPE_OPAQUE | CTLFLAG_RD | CTLFLAG_MPSAFE,
+			    OID_AUTO, ksp->ks_name, CTLTYPE_OPAQUE | CTLFLAG_RD
+			    | CTLFLAG_MPSAFE | CTLFLAG_SKIP,
 			    ksp, 0, kstat_sysctl_raw, "", ksp->ks_name);
 		}
 		break;