SUNRPC: Add a trace point in svc_xprt_deferred_close

The trace point in svc_xprt_close() reports only some local close
requests. Try to capture more local close requests.

Note that "trace-cmd record -T -e sunrpc:svc_xprt_close" will
neatly capture the identity of the caller requesting the close.

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