fanotify: fix FAN_Q_OVERFLOW case of fanotify_read()

If the FAN_Q_OVERFLOW bit set in event->mask, the fanotify event
metadata will not contain a valid file descriptor, but
copy_event_to_user() didn't check for that, and unconditionally does a
fd_install() on the file descriptor.

Which in turn will cause a BUG_ON() in __fd_install().

Introduced by commit 352e3b24 ("fanotify: sanitize failure exits in
copy_event_to_user()")

Mea culpa - missed that path ;-/
Reported-by: Alex Shi <lkml.alex@gmail.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/notify/fanotify/fanotify_user.c

@@ -258,7 +258,8 @@ static ssize_t copy_event_to_user(struct fsnotify_group *group,
 	if (ret)
 		goto out_close_fd;
 
-	fd_install(fd, f);
+	if (fd != FAN_NOFD)
+		fd_install(fd, f);
 	return fanotify_event_metadata.event_len;
 
 out_close_fd: