SUNRPC: Add a ->free_slot transport callout

Refactor: xprtrdma needs to have better control over when RPCs are
awoken from the backlog queue, so replace xprt_free_slot with a
transport op callout.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>
diff --git a/net/sunrpc/xprtrdma/svc_rdma_backchannel.c b/net/sunrpc/xprtrdma/svc_rdma_backchannel.c
index a73632c..1035516 100644
--- a/net/sunrpc/xprtrdma/svc_rdma_backchannel.c
+++ b/net/sunrpc/xprtrdma/svc_rdma_backchannel.c
@@ -273,6 +273,7 @@ static const struct rpc_xprt_ops xprt_rdma_bc_procs = {
 	.reserve_xprt		= xprt_reserve_xprt_cong,
 	.release_xprt		= xprt_release_xprt_cong,
 	.alloc_slot		= xprt_alloc_slot,
+	.free_slot		= xprt_free_slot,
 	.release_request	= xprt_release_rqst_cong,
 	.buf_alloc		= xprt_rdma_bc_allocate,
 	.buf_free		= xprt_rdma_bc_free,