[9P] net/9p/trans_fd.c: remove unused variable

The variable cb is initialized but never used otherwise.

The semantic patch that makes this change is as follows:
(http://www.emn.fr/x-info/coccinelle/)

// <smpl>
@@
type T;
identifier i;
constant C;
@@

(
extern T i;
|
- T i;
  <+... when != i
- i = C;
  ...+>
)
// </smpl>
Signed-off-by: Julia Lawall <julia@diku.dk>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/9p/trans_fd.c

@@ -861,7 +861,6 @@ static void p9_mux_free_request(struct p9_conn *m, struct p9_req *req)
 
 static void p9_mux_flush_cb(struct p9_req *freq, void *a)
 {
-	p9_conn_req_callback cb;
 	int tag;
 	struct p9_conn *m;
 	struct p9_req *req, *rreq, *rptr;
@@ -872,7 +871,6 @@ static void p9_mux_flush_cb(struct p9_req *freq, void *a)
 		freq->tcall->params.tflush.oldtag);
 
 	spin_lock(&m->lock);
-	cb = NULL;
 	tag = freq->tcall->params.tflush.oldtag;
 	req = NULL;
 	list_for_each_entry_safe(rreq, rptr, &m->req_list, req_list) {