Don't allow dnode allocation if dn_holds != 0

This patch simply fixes a small bug where dnode_hold_impl() could
attempt to allocate a dnode that was in the process of being freed,
but which still had active references. This patch simply adds the
required check.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tom Caputi <tcaputi@datto.com>
Closes #8249

module/zfs/dnode.c

@@ -1106,6 +1106,7 @@ dnode_check_slots_free(dnode_children_t *children, int idx, int slots)
 		} else if (DN_SLOT_IS_PTR(dn)) {
 			mutex_enter(&dn->dn_mtx);
 			boolean_t can_free = (dn->dn_type == DMU_OT_NONE &&
+			    zfs_refcount_is_zero(&dn->dn_holds) &&
 			    !DNODE_IS_DIRTY(dn));
 			mutex_exit(&dn->dn_mtx);