vfs: stop counting on gcc not messing with mnt_expiry_mark if not asked

So happens it already was not doing it, but there is no need to "hope"
as indicated in the comment.

No changes in generated assembly.

Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Link: https://lore.kernel.org/r/20231004111916.728135-3-mjguzik@gmail.com


Signed-off-by: Christian Brauner <brauner@kernel.org>