virtio_p9fs: Fix kernel panic on module unload

The virtio_p9fs module event handler can be invoked multiple times.
Previously, this caused p9_init_zones() and p9_register_trans() to be
executed multiple times, leaking UMA zones and corrupting the transport
list. During module unload, p9_destroy_zones() was also called multiple
times on the same zone pointers, triggering a duplicate free kernel panic
in uma_zdestroy().

This patch introduces a static reference counter in vt9p_modevent() to
ensure the zones and transports are only initialized and destroyed exactly
once, aligning with the approach used by other virtio drivers like vtnet.

Reviewed by:	kib, markj
MFC after:	1 week
Differential Revision: https://reviews.freebsd.org/D56497

sys/dev/virtio/p9fs/virtio_p9fs.c

@@ -464,16 +464,22 @@ static int
 vt9p_modevent(module_t mod, int type, void *unused)
 {
 	int error;
+	static int loaded = 0;
 
 	error = 0;
 
 	switch (type) {
 	case MOD_LOAD:
-		p9_init_zones();
-		p9_register_trans(&vt9p_trans);
+		if (loaded++ == 0) {
+			p9_init_zones();
+			p9_register_trans(&vt9p_trans);
+		}
 		break;
 	case MOD_UNLOAD:
-		p9_destroy_zones();
+		if (--loaded == 0) {
+			p9_unregister_trans(&vt9p_trans);
+			p9_destroy_zones();
+		}
 		break;
 	case MOD_SHUTDOWN:
 		break;