bsdinstall: Make ZFS min_auto_ashift adjustment persistent

Reported by:	feld
Reviewed by:	dteske, tsoome
MFC after:	3 days
Sponsored by:	ScaleEngine Inc.
Differential Revision:	https://reviews.freebsd.org/D10895

usr.sbin/bsdinstall/scripts/zfsboot

@@ -1443,6 +1443,12 @@ zfs_create_boot()
 	             'kern.geom.label.gptid.enable=\"0\"' \
 	             $BSDINSTALL_TMPBOOT/loader.conf.zfs || return $FAILURE
 
+	if [ "$ZFSBOOT_FORCE_4K_SECTORS" ]; then
+		f_eval_catch $funcname echo "$ECHO_APPEND" \
+	             'vfs.zfs.min_auto_ashift=12' \
+	             $BSDINSTALL_TMPBOOT/loader.conf.zfs || return $FAILURE
+	fi
+
 	if [ "$ZFSBOOT_SWAP_MIRROR" ]; then
 		f_eval_catch $funcname echo "$ECHO_APPEND" \
 		             'geom_mirror_load=\"YES\"' \