net/9p: fix client code to fail more gracefully on protocol error

There was a BUG_ON to protect against a bad id which could be dealt with
more gracefully.
Reported-by: Natalie Orlin <norlin@us.ibm.com>
Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>

net/9p/client.c

@@ -280,7 +280,8 @@ struct p9_req_t *p9_tag_lookup(struct p9_client *c, u16 tag)
 	 * buffer to read the data into */
 	tag++;
 
-	BUG_ON(tag >= c->max_tag);
+	if(tag >= c->max_tag) 
+		return NULL;
 
 	row = tag / P9_ROW_MAXTAG;
 	col = tag % P9_ROW_MAXTAG;