fsconfig: ensure that dirfd is set to aux

The code in fs_param_is_fd() expects param->dirfd to be set to the fd
that was used to set param->file to initialize result->uint_32. So make
sure it's set so users like autofs using FSCONFIG_SET_FD with the new
mount api can rely on this to be set to the correct value.

Link: https://lore.kernel.org/lkml/20230922-vorbringen-spaghetti-946729122076@brauner
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/fsopen.c

@@ -465,6 +465,7 @@ SYSCALL_DEFINE5(fsconfig,
 		param.file = fget(aux);
 		if (!param.file)
 			goto out_key;
+		param.dirfd = aux;
 		break;
 	default:
 		break;