lib/vsprintf: Fix %pfwf when current node refcount == 0

A refcount issue can appeared in __fwnode_link_del() due to the
pr_debug() call:
  WARNING: CPU: 0 PID: 901 at lib/refcount.c:25 refcount_warn_saturate+0xe5/0x110
  Call Trace:
  <TASK>
  ...
  of_node_get+0x1e/0x30
  of_fwnode_get+0x28/0x40
  fwnode_full_name_string+0x34/0x90
  fwnode_string+0xdb/0x140
  ...
  vsnprintf+0x17b/0x630
  ...
  __fwnode_link_del+0x25/0xa0
  fwnode_links_purge+0x39/0xb0
  of_node_release+0xd9/0x180
  ...

Indeed, an fwnode (of_node) is being destroyed and so, of_node_release()
is called because the of_node refcount reached 0.
From of_node_release() several function calls are done and lead to
a pr_debug() calls with %pfwf to print the fwnode full name.
The issue is not present if we change %pfwf to %pfwP.

To print the full name, %pfwf iterates over the current node and its
parents and obtain/drop a reference to all nodes involved.

In order to allow to print the full name (%pfwf) of a node while it is
being destroyed, do not obtain/drop a reference to this current node.

Fixes: a92eb762 ("lib/vsprintf: Make use of fwnode API to obtain node names and separators")
Cc: stable@vger.kernel.org
Signed-off-by: Herve Codina <herve.codina@bootlin.com>
Reviewed-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20231114152655.409331-1-herve.codina@bootlin.com

lib/vsprintf.c

@@ -2111,14 +2111,19 @@ char *fwnode_full_name_string(struct fwnode_handle *fwnode, char *buf,
 
 	/* Loop starting from the root node to the current node. */
 	for (depth = fwnode_count_parents(fwnode); depth >= 0; depth--) {
-		struct fwnode_handle *__fwnode =
-			fwnode_get_nth_parent(fwnode, depth);
+		/*
+		 * Only get a reference for other nodes (i.e. parent nodes).
+		 * fwnode refcount may be 0 here.
+		 */
+		struct fwnode_handle *__fwnode = depth ?
+			fwnode_get_nth_parent(fwnode, depth) : fwnode;
 
 		buf = string(buf, end, fwnode_get_name_prefix(__fwnode),
 			     default_str_spec);
 		buf = string(buf, end, fwnode_get_name(__fwnode),
 			     default_str_spec);
 
+		if (depth)
 			fwnode_handle_put(__fwnode);
 	}