Fix build with CONFIG_GCC_PLUGIN_RANDSTRUCT

fs/zfs/zfs/metaslab.c:1055:2: error: positional initialization of field
in ‘struct’ declared with ‘designated_init’ attribute
[-Werror=designated-init]
  metaslab_rt_remove,

Signed-off-by: Georgy Yakovlev <ya@sysdump.net>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Closes: #7069

module/zfs/metaslab.c

@@ -1049,11 +1049,11 @@ metaslab_rt_vacate(range_tree_t *rt, void *arg)
 }
 
 static range_tree_ops_t metaslab_rt_ops = {
-	metaslab_rt_create,
-	metaslab_rt_destroy,
-	metaslab_rt_add,
-	metaslab_rt_remove,
-	metaslab_rt_vacate
+	.rtop_create = metaslab_rt_create,
+	.rtop_destroy = metaslab_rt_destroy,
+	.rtop_add = metaslab_rt_add,
+	.rtop_remove = metaslab_rt_remove,
+	.rtop_vacate = metaslab_rt_vacate
 };
 
 /*