Port the __I_SYNC mechanism to use the new var event mechanism.

Link: https://lore.kernel.org/r/20240823-work-i_state-v3-3-5cd5fd207a57@kernel.orgReviewed-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>