Add ida_destroy in zvol_fini to fix memleak

User of ida needs to call ida_destroy after using it. Otherwise
ida->free_bitmap and/or other stuff may leak.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Chunwei Chen <david.chen@osnexus.com>
Closes #5484

module/zfs/zvol.c

@@ -2085,6 +2085,8 @@ zvol_fini(void)
 
 	list_destroy(&zvol_state_list);
 	mutex_destroy(&zvol_state_lock);
+
+	ida_destroy(&zvol_ida);
 }
 
 /* BEGIN CSTYLED */