fsnotify: Do not generate events for O_PATH file descriptors

Currently we will not generate FS_OPEN events for O_PATH file
descriptors but we will generate FS_CLOSE events for them. This is
asymmetry is confusing. Arguably no fsnotify events should be generated
for O_PATH file descriptors as they cannot be used to access or modify
file content, they are just convenient handles to file objects like
paths. So fix the asymmetry by stopping to generate FS_CLOSE for O_PATH
file descriptors.

Cc: <[e-mail]>
Signed-off-by: Jan Kara <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Reviewed-by: Amir Goldstein <[e-mail]>
Signed-off-by: Christian Brauner <[e-mail]>

include/linux/fsnotify.h

···
 {
 	const struct path *path;
 
-	if (file->f_mode & FMODE_NONOTIFY)
+	/*
+	 * FMODE_NONOTIFY are fds generated by fanotify itself which should not
+	 * generate new events. We also don't want to generate events for
+	 * FMODE_PATH fds (involves open & close events) as they are just
+	 * handle creation / destruction events and not "real" file events.
+	 */
+	if (file->f_mode & (FMODE_NONOTIFY | FMODE_PATH))
 		return 0;
 
 	path = &file->f_path;