nsproxy: restore EINVAL for non-namespace file descriptor

The LTP testsuite reported a regression where users would now see EBADF
returned instead of EINVAL when an fd was passed that referred to an open
file but the file was not a nsfd. Fix this by continuing to report EINVAL.

Reported-by: kernel test robot <[e-mail]>
Cc: Jan Stancek <[e-mail]>
Cc: Cyril Hrubis <[e-mail]>
Link: https://lore.kernel.org/lkml/20200615085836.GR12456@shao2-debian
Fixes: 303cc571d107 ("nsproxy: attach to namespaces via pidfds")
Signed-off-by: Christian Brauner <[e-mail]>

kernel/nsproxy.c

···
 	} else if (!IS_ERR(pidfd_pid(file))) {
 		err = check_setns_flags(flags);
 	} else {
-		err = -EBADF;
+		err = -EINVAL;
 	}
 	if (err)
 		goto out;