config: fix one-node dumpconfig, add dm_config_write_one_node

A regression introduced in 2.02.89 (11e520256b3005ed813ce83f8770aaab74edef3f)
caused the lvm dumpconfig <node> to print out
the node as well as its subsequent siblings.
The information about "only_one" mode got lost.

Before this patch (just an example node):
  # lvm dumpconfig global/use_lvmetad
  use_lvmetad=1
  thin_check_executable="/usr/sbin/thin_check"
  thin_check_options="-q"
  (...all nodes to the end of the section)

With this patch applied:
   # lvm dumpconfig global/use_lvmetad
   use_lvmetad=1

1 files changed, 13 insertions, 2 deletions

diff --git a/libdm/libdm-config.c b/libdm/libdm-config.c
index d9d3882f2..845749797 100644
--- a/libdm/libdm-config.c
+++ b/libdm/libdm-config.c
@@ -331,14 +331,15 @@ static int _write_config(const struct dm_config_node *n, int only_one,
 	return 1;
 }
 
-int dm_config_write_node(const struct dm_config_node *cn, dm_putline_fn putline, void *baton)
+static int _write_node(const struct dm_config_node *cn, int only_one,
+		       dm_putline_fn putline, void *baton)
 {
 	struct output_line outline;
 	if (!(outline.mem = dm_pool_create("config_line", 1024)))
 		return_0;
 	outline.putline = putline;
 	outline.putline_baton = baton;
-	if (!_write_config(cn, 0, &outline, 0)) {
+	if (!_write_config(cn, only_one, &outline, 0)) {
 		dm_pool_destroy(outline.mem);
 		return_0;
 	}
@@ -346,6 +347,16 @@ int dm_config_write_node(const struct dm_config_node *cn, dm_putline_fn putline,
 	return 1;
 }
 
+int dm_config_write_one_node(const struct dm_config_node *cn, dm_putline_fn putline, void *baton)
+{
+	return _write_node(cn, 1, putline, baton);
+}
+
+int dm_config_write_node(const struct dm_config_node *cn, dm_putline_fn putline, void *baton)
+{
+	return _write_node(cn, 0, putline, baton);
+}
+
 
 /*
  * parser