svcrdma: Use standard Linux lists for context cache

Replace the one-off linked list implementation used to implement the
context cache with the standard Linux list_head lists. Add a context
counter to catch resource leaks. A WARN_ON will be added later to
ensure that we've freed all contexts.

Signed-off-by: Tom Tucker <tom@opengridcomputing.com>
diff --git a/include/linux/sunrpc/svc_rdma.h b/include/linux/sunrpc/svc_rdma.h
index c447c41..7014390 100644
--- a/include/linux/sunrpc/svc_rdma.h
+++ b/include/linux/sunrpc/svc_rdma.h
@@ -72,7 +72,7 @@
  */
 struct svc_rdma_op_ctxt {
 	struct svc_rdma_op_ctxt *read_hdr;
-	struct svc_rdma_op_ctxt *next;
+	struct list_head free_list;
 	struct xdr_buf arg;
 	struct list_head dto_q;
 	enum ib_wr_opcode wr_op;
@@ -104,7 +104,8 @@
 
 	struct ib_pd         *sc_pd;
 
-	struct svc_rdma_op_ctxt  *sc_ctxt_head;
+	atomic_t	     sc_ctxt_used;
+	struct list_head     sc_ctxt_free;
 	int		     sc_ctxt_cnt;
 	int		     sc_ctxt_bump;
 	int		     sc_ctxt_max;