vfs: Convert BUG_ON to WARN_ON_ONCE in open_last_lookups

The calling code actually handles -ECHILD, so this BUG_ON
can be converted to WARN_ON_ONCE.
Signed-off-by: Bernd Schubert <bschubert@ddn.com>
Link: https://lore.kernel.org/r/20231023184718.11143-1-bschubert@ddn.com
Cc: Christian Brauner <brauner@kernel.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Amir Goldstein <amir73il@gmail.com>
Cc: Dharmendra Singh <dsingh@ddn.com>
Cc: Miklos Szeredi <miklos@szeredi.hu>
Cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/namei.c

@@ -3516,7 +3516,8 @@ static const char *open_last_lookups(struct nameidata *nd,
 		if (likely(dentry))
 			goto finish_lookup;
 
-		BUG_ON(nd->flags & LOOKUP_RCU);
+		if (WARN_ON_ONCE(nd->flags & LOOKUP_RCU))
+			return ERR_PTR(-ECHILD);
 	} else {
 		/* create side of things */
 		if (nd->flags & LOOKUP_RCU) {