I propose we add an HSM request flag to indicate that an action has been triggered by the kernel and a user process is currently blocked on this action.

AFAICT, the hsm_user_request.hr_flags field is currently unused by the kernel or any of the user tools or tests in the tree, so we can use this field for this flag. I also don't see any flags defined, so perhaps it could be something like HR_BLOCKING or HR_SYNC.

It looks like ll_layout_restore() is used when we need to trigger to restore from the kernel, so that is probably the right place to put this.