eventfs: Fix WARN_ON() in create_file_dentry()

As the comment right above a WARN_ON() in create_file_dentry() states:

  * Note, with the mutex held, the e_dentry cannot have content
  * and the ei->is_freed be true at the same time.

But the WARN_ON() only has:

  WARN_ON_ONCE(ei->is_free);

Where to match the comment (and what it should actually do) is:

  dentry = *e_dentry;
  WARN_ON_ONCE(dentry && ei->is_free)

Also in that case, set dentry to NULL (although it should never happen).

Link: https://lore.kernel.org/linux-trace-kernel/20231024123628.62b88755@gandalf.local.home

Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Fixes: 5790b1fb ("eventfs: Remove eventfs_file and just use eventfs_inode")
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

fs/tracefs/event_inode.c

@@ -264,8 +264,9 @@ create_file_dentry(struct eventfs_inode *ei, struct dentry **e_dentry,
 		 * Note, with the mutex held, the e_dentry cannot have content
 		 * and the ei->is_freed be true at the same time.
 		 */
-		WARN_ON_ONCE(ei->is_freed);
 		dentry = *e_dentry;
+		if (WARN_ON_ONCE(dentry && ei->is_freed))
+			dentry = NULL;
 		/* The lookup does not need to up the dentry refcount */
 		if (dentry && !lookup)
 			dget(dentry);