When we are running low on vnodes, there is currently no way to ask other

subsystems to release some vnodes. Implement backpressure based on
vfs_lowvnodes event (similar to vm_lowmem for memory).

sys/kern/vfs_subr.c

@@ -736,6 +736,7 @@ vnlru_proc(void)
 		}
 		mtx_unlock(&mountlist_mtx);
 		if (done == 0) {
+			EVENTHANDLER_INVOKE(vfs_lowvnodes, desiredvnodes / 10);
 #if 0
 			/* These messages are temporary debugging aids */
 			if (vnlru_nowhere < 5)

sys/sys/eventhandler.h

@@ -167,6 +167,10 @@ typedef void (*vm_lowmem_handler_t)(void *, int);
 #define	LOWMEM_PRI_DEFAULT	EVENTHANDLER_PRI_FIRST
 EVENTHANDLER_DECLARE(vm_lowmem, vm_lowmem_handler_t);
 
+/* Low vnodes event */
+typedef void (*vfs_lowvnodes_handler_t)(void *, int);
+EVENTHANDLER_DECLARE(vfs_lowvnodes, vfs_lowvnodes_handler_t);
+
 /*
  * Process events
  * process_fork and exit handlers are called without Giant.