Fix pool creation with feature@allocation_classes disabled

When "feature@allocation_classes" is not enabled on the pool no vdev
with "special" or "dedup" allocation type should be allowed to exist in
the vdev tree.

Reviewed-by: Pavel Zakharov <pavel.zakharov@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #9427 
Closes #9429

tests/zfs-tests/tests/functional/alloc_class/alloc_class_001_pos.ksh

@@ -20,7 +20,8 @@
 
 #
 # DESCRIPTION:
-#	Creating a pool with a special device succeeds.
+#	Creating a pool with a special device succeeds, but only if
+#	"feature@allocation_classes" is enabled.
 #
 
 verify_runnable "global"
@@ -31,6 +32,9 @@ log_assert $claim
 log_onexit cleanup
 
 log_must disk_setup
+for type in special dedup; do
+	log_mustnot zpool create -d $TESTPOOL $CLASS_DISK0 $type $CLASS_DISK1
+done
 log_must zpool create $TESTPOOL raidz $ZPOOL_DISKS special mirror \
     $CLASS_DISK0 $CLASS_DISK1
 log_must display_status "$TESTPOOL"