9p/trans_fd: Correct function name p9_mux_destroy() in the kerneldoc

Fix the following W=1 kernel build warning(s):

 net/9p/trans_fd.c:881: warning: expecting prototype for p9_mux_destroy(). Prototype was for p9_conn_destroy() instead
Reported-by: NHulk Robot <hulkci@huawei.com>
Signed-off-by: NXiongfeng Wang <wangxiongfeng2@huawei.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/9p/trans_fd.c

@@ -872,7 +872,7 @@ static int p9_socket_open(struct p9_client *client, struct socket *csocket)
 }
 
 /**
- * p9_mux_destroy - cancels all pending requests of mux
+ * p9_conn_destroy - cancels all pending requests of mux
  * @m: mux to destroy
  *
  */