eventfs: Don't return NULL in eventfs_create_dir()

Commit 77a06c33a2 ("eventfs: Test for ei->is_freed when accessing
ei->dentry") added another check, testing if the parent was freed after
we released the mutex. If so, the function returns NULL. However, all
callers expect it to either return a valid pointer or an error pointer,
at least since commit 5264a2f4bb ("tracing: Fix a NULL vs IS_ERR() bug
in event_subsystem_dir()"). Returning NULL will therefore fail the error
condition check in the caller.

Fix this by substituting the NULL return value with a fitting error
pointer.

Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: stable@vger.kernel.org
Fixes: 77a06c33a2 ("eventfs: Test for ei->is_freed when accessing ei->dentry")
Link: https://lore.kernel.org/20240723122522.2724-1-minipli@grsecurity.net
Reviewed-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Ajay Kaher <ajay.kaher@broadcom.com>
Signed-off-by: Mathias Krause <minipli@grsecurity.net>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

fs/tracefs/event_inode.c

@@ -736,7 +736,7 @@ struct eventfs_inode *eventfs_create_dir(const char *name, struct eventfs_inode
 	/* Was the parent freed? */
 	if (list_empty(&ei->list)) {
 		cleanup_ei(ei);
-		ei = NULL;
+		ei = ERR_PTR(-EBUSY);
 	}
 	return ei;
 }