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.