Throttle switching kernel acceleration

Creating/destroying interfaces etc. does take some time. When e.g. receiving
a lot of sessions as new slave, we don't want to stay stuck creating
hundreds of interfaces while we are already receiving control messages
that we have to forward to master not too late. Switching kernel
acceleration can wait a bit most of the time.

l2tpns.h

@@ -452,6 +452,9 @@ typedef struct
 	// time in milliseconds of the last fragment.
 	uint64_t prev_time;
 
+	// Pending kernel switch
+	int needs_switch;
+
 	// l2tp PPPoL2TP socket
 	int pppox_fd;
 	struct pppol2tp_ioc_stats last_stats;