sunrpc: Add static trace point to report result of RPC ping

This information can help track down local misconfiguration issues
as well as network partitions and unresponsive servers.

There are several ways to send a ping, and with transport multi-
plexing, the exact rpc_xprt that is used is sometimes not known by
the upper layer. The rpc_xprt pointer passed to the trace point
call also has to be RCU-safe.

I found a spot inside the client FSM where an rpc_xprt pointer is
always available and safe to use.

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