2*(void *) atomic swap?

[Maxime Villard <max%m00nbsd.net@localhost>]

Do we have a magic function that can perform two atomic_swap_ptr()
atomically?

In kern/subr_kmem.c::kmem_guard_free(), the code used to be

	rotor = atomic_inc_uint_nv(&kg->kg_rotor) % kg->kg_depth;
	va = (vaddr_t)atomic_swap_ptr(&kg->kg_fifo[rotor], (void *)va);
	if (va != 0) {
		vmem_free(kg->kg_vmem, va, size);
	}

The va was inserted into the FIFO atomically, and that was fine. Now
we also need to register the size of the va into that FIFO, which means
we have to perform another atomic_swap_ptr(), but there's obviously a
window in the meantime.

I don't want to use a global lock; it may slow down the system..