Rename _lock to _tx_gc_lock

drivers/virtio-net.cc

@@ -367,7 +367,7 @@ namespace virtio {
 
     void virtio_net::tx_gc()
     {
-        with_lock(_lock, [=] {
+        with_lock(_tx_gc_lock, [=] {
             u32 len;
             virtio_net_req * req;
 

drivers/virtio-net.hh

@@ -261,7 +261,7 @@ namespace virtio {
         struct ifnet* _ifn;
 
         // tx gc lock that can be called by the gc thread or the tx xmitter
-        mutex _lock;
+        mutex _tx_gc_lock;
     };
 }