xprtrdma: Fix a BUG when tracing is enabled with NFSv4.1 on RDMA

A backchannel reply does not set task->tk_client.

Fixes: 0c77668d ("SUNRPC: Introduce trace points in ... ")
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>