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>