SUNRPC: Generalize the RPC buffer release API

xprtrdma needs to allocate the Call and Reply buffers separately.
TBH, the reliance on using a single buffer for the pair of XDR
buffers is transport implementation-specific.

Instead of passing just the rq_buffer into the buf_free method, pass
the task structure and let buf_free take care of freeing both
XDR buffers at once.

There's a micro-optimization here. In the common case, both
xprt_release and the transport's buf_free method were checking if
rq_buffer was NULL. Now the check is done only once per RPC.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

net/sunrpc/xprtrdma/svc_rdma_backchannel.c

@@ -186,7 +186,7 @@ xprt_rdma_bc_allocate(struct rpc_task *task)
 }
 
 static void
-xprt_rdma_bc_free(void *buffer)
+xprt_rdma_bc_free(struct rpc_task *task)
 {
 	/* No-op: ctxt and page have already been freed. */
 }