drbd: register peer requests on read_ee early

Initialize peer_request with timestamp and proper empty list head.
Add peer_request to list early, so debugfs can find this request and
report it as "preparing", even if we sleep before we actually submit it.

Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>