drbd: base completion and destruction of requests on ref counts

cherry-picked and adapted from drbd 9 devel branch

The logic for when to get or put a reference is in mod_rq_state().

To not get confused in the freeze/thaw respectively resend/restart
paths, or when cleaning up requests waiting for P_BARRIER_ACK, this
also introduces additional state flags:
RQ_COMPLETION_SUSP, and RQ_EXP_BARR_ACK.

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

drivers/block/drbd/drbd_int.h

@@ -575,13 +575,14 @@ struct drbd_request {
 
 	struct list_head tl_requests; /* ring list in the transfer log */
 	struct bio *master_bio;       /* master bio pointer */
-	unsigned long rq_state; /* see comments above _req_mod() */
 	unsigned long start_time;
 
 	/* once it hits 0, we may complete the master_bio */
 	atomic_t completion_ref;
 	/* once it hits 0, we may destroy this drbd_request object */
 	struct kref kref;
+
+	unsigned rq_state; /* see comments above _req_mod() */
 };
 
 struct drbd_epoch {

drivers/block/drbd/drbd_main.c

@@ -210,7 +210,7 @@ void tl_release(struct drbd_tconn *tconn, unsigned int barrier_nr,
 	/* find latest not yet barrier-acked write request,
 	 * count writes in its epoch. */
 	list_for_each_entry(r, &tconn->transfer_log, tl_requests) {
-		const unsigned long s = r->rq_state;
+		const unsigned s = r->rq_state;
 		if (!req) {
 			if (!(s & RQ_WRITE))
 				continue;