lightnvm: refactor end_io functions for sync

To implement sync I/O support within the LightNVM core, the end_io
functions are refactored to take an end_io function pointer instead of
testing for initialized media manager, followed by calling its end_io
function.

Sync I/O can then be implemented using a callback that signal I/O
completion. This is similar to the logic found in blk_to_execute_io().
By implementing it this way, the underlying device I/Os submission logic
is abstracted away from core, targets, and media managers.

Signed-off-by: Matias Bjørling <m@bjorling.me>
Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/drivers/lightnvm/rrpc.c b/drivers/lightnvm/rrpc.c
index 748cab49..661c6f3 100644
--- a/drivers/lightnvm/rrpc.c
+++ b/drivers/lightnvm/rrpc.c
@@ -642,7 +642,7 @@
 	}
 }
 
-static int rrpc_end_io(struct nvm_rq *rqd, int error)
+static void rrpc_end_io(struct nvm_rq *rqd, int error)
 {
 	struct rrpc *rrpc = container_of(rqd->ins, struct rrpc, instance);
 	struct rrpc_rq *rrqd = nvm_rq_to_pdu(rqd);
@@ -655,7 +655,7 @@
 	bio_put(rqd->bio);
 
 	if (rrqd->flags & NVM_IOTYPE_GC)
-		return 0;
+		return;
 
 	rrpc_unlock_rq(rrpc, rqd);
 
@@ -665,8 +665,6 @@
 		nvm_dev_dma_free(rrpc->dev, rqd->metadata, rqd->dma_metadata);
 
 	mempool_free(rqd, rrpc->rq_pool);
-
-	return 0;
 }
 
 static int rrpc_read_ppalist_rq(struct rrpc *rrpc, struct bio *bio,