drbd: simplify retry path of failed READ requests

If a local or remote READ request fails, just push it back to the retry
workqueue.  It will re-enter __drbd_make_request, and be re-assigned to
a suitable local or remote path, or failed, if we do not have access to
good data anymore.

This obsoletes w_read_retry_remote(),
and eliminates two goto...retry blocks in __req_mod()

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