securityfs: fix securityfs_create_dir comment

If there is an error creating a directory with securityfs_create_dir,
the error is propagated via ERR_PTR but the function comment claims that
NULL is returned.

This is a similar commit to 88e6c94cda322ff2b32f72bb8d96f9675cdad8aa
("fix long-broken securityfs_create_file comment") that did not fix
securityfs_create_dir comment at the same time.
Signed-off-by: Laurent Georget <laurent.georget@supelec.fr>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

security/inode.c

@@ -156,12 +156,11 @@ EXPORT_SYMBOL_GPL(securityfs_create_file);
  * This function returns a pointer to a dentry if it succeeds.  This
  * pointer must be passed to the securityfs_remove() function when the file is
  * to be removed (no automatic cleanup happens if your module is unloaded,
- * you are responsible here).  If an error occurs, %NULL will be returned.
+ * you are responsible here).  If an error occurs, the function will return
+ * the error value (via ERR_PTR).
  *
  * If securityfs is not enabled in the kernel, the value %-ENODEV is
- * returned.  It is not wise to check for this value, but rather, check for
- * %NULL or !%NULL instead as to eliminate the need for #ifdef in the calling
- * code.
+ * returned.
  */
 struct dentry *securityfs_create_dir(const char *name, struct dentry *parent)
 {