RDS: Assume recv->r_frag is always NULL in refill_one()

refill_one() should never be called on a recv struct that
doesn't need a new r_frag allocated. Add a WARN and remove
conditional around r_frag alloc code.

Also, add a comment to explain why r_ibinc may or may not
need refilling.
Signed-off-by: Andy Grover <andy.grover@oracle.com>

net/rds/ib_recv.c

@@ -126,6 +126,10 @@ static int rds_ib_recv_refill_one(struct rds_connection *conn,
 	struct ib_sge *sge;
 	int ret = -ENOMEM;
 
+	/*
+	 * ibinc was taken from recv if recv contained the start of a message.
+	 * recvs that were continuations will still have this allocated.
+	 */
 	if (!recv->r_ibinc) {
 		if (!atomic_add_unless(&rds_ib_allocation, 1, rds_ib_sysctl_max_recv_allocation)) {
 			rds_ib_stats_inc(s_ib_rx_alloc_limit);
@@ -140,7 +144,7 @@ static int rds_ib_recv_refill_one(struct rds_connection *conn,
 		rds_inc_init(&recv->r_ibinc->ii_inc, conn, conn->c_faddr);
 	}
 
-	if (!recv->r_frag) {
+	WARN_ON(recv->r_frag); /* leak! */
 	recv->r_frag = kmem_cache_alloc(rds_ib_frag_slab, GFP_NOWAIT);
 	if (!recv->r_frag)
 		goto out;
@@ -153,7 +157,6 @@ static int rds_ib_recv_refill_one(struct rds_connection *conn,
 		recv->r_frag = NULL;
 		goto out;
 	}
-	}
 
 	ret = ib_dma_map_sg(ic->i_cm_id->device, &recv->r_frag->f_sg,
 			    1, DMA_FROM_DEVICE);