svcrpc: assume svc_delete_xprt() called only once

As long as DEAD exports are left BUSY, and svc_delete_xprt is called
only with BUSY held, then svc_delete_xprt() will never be called on an
xprt that is already DEAD.
Signed-off-by: NJ. Bruce Fields <bfields@redhat.com>

net/sunrpc/svc_xprt.c

@@ -903,7 +903,7 @@ void svc_delete_xprt(struct svc_xprt *xprt)
 
 	/* Only do this once */
 	if (test_and_set_bit(XPT_DEAD, &xprt->xpt_flags))
-		return;
+		BUG();
 
 	dprintk("svc: svc_delete_xprt(%p)\n", xprt);
 	xprt->xpt_ops->xpo_detach(xprt);