sunrpc: Fix latency trace point crashes

If the rpc_task survived longer than the transport, task->tk_xprt
points to freed memory by the time rpc_count_iostats_metrics runs.
Replace the references to task->tk_xprt with references to the
task's tk_client.

Reported-by:  <syzbot+27db1f90e2b972a5f2d3@syzkaller.appspotmail.com>
Fixes: 40bf7eb3

 ('sunrpc: Add static trace point to report ...')
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>