svcrpc: svc_close_xprt comment

Neil Brown had to explain to me why we do this here; record the answer
for posterity.
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

net/sunrpc/svc_xprt.c

@@ -931,7 +931,12 @@ void svc_close_xprt(struct svc_xprt *xprt)
 	if (test_and_set_bit(XPT_BUSY, &xprt->xpt_flags))
 		/* someone else will have to effect the close */
 		return;
-
+	/*
+	 * We expect svc_close_xprt() to work even when no threads are
+	 * running (e.g., while configuring the server before starting
+	 * any threads), so if the transport isn't busy, we delete
+	 * it ourself:
+	 */
 	svc_delete_xprt(xprt);
 }
 EXPORT_SYMBOL_GPL(svc_close_xprt);