proc: remove useless d_is_dir() check

Remove the d_is_dir() check from tgid_pidfd_to_pid().

It is pointless since you should never get &proc_tgid_base_operations
for f_op on a non-directory.
Suggested-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Christian Brauner <christian@brauner.io>

fs/proc/base.c

@@ -3077,8 +3077,7 @@ static const struct file_operations proc_tgid_base_operations = {
 
 struct pid *tgid_pidfd_to_pid(const struct file *file)
 {
-	if (!d_is_dir(file->f_path.dentry) ||
-	    (file->f_op != &proc_tgid_base_operations))
+	if (file->f_op != &proc_tgid_base_operations)
 		return ERR_PTR(-EBADF);
 
 	return proc_pid(file_inode(file));