[libj6] Account for double-mapped ring buffers in j6::channel

When keeping track of addresses to give to the kernel for channel shared
memory, double the channel's size since the kernel will double-map the
area as a ring buffer.

src/libraries/j6/channel.cpp

@@ -54,7 +54,7 @@ channel::create(size_t size)
 
     util::scoped_lock lock {addr_spinlock};
     uintptr_t addr = channel_addr;
-    channel_addr += size;
+    channel_addr += size * 2; // account for ring buffer virtual space doubling
     lock.release();
 
     result = j6_vma_create_map(&vma, size, addr, j6_vm_flag_write|j6_vm_flag_ring);