SUNRPC: add xrpt id to rpc_stats_latency tracepoint

In order to get the latency per xprt under the same clientid this patch
adds xprt_id to the tracepoint output.

Signed-off-by: Olga Kornievskaia <kolga@netapp.com>
Tested-by: Chen Hanxiao <chenhx.fnst@fujitsu.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>