Kernel: Add KBuffer, a simple byte buffer backed by kernel-only memory

This memory is not accessible to userspace and comes from the kernel
page allocator, not from the kmalloc heap. This makes it ideal for
larger allocations.

Kernel/KBuffer.h

@@ -0,0 +1,37 @@
+#pragma once
+
+#include <AK/Assertions.h>
+#include <Kernel/VM/MemoryManager.h>
+#include <Kernel/VM/Region.h>
+
+class KBuffer : public RefCounted<KBuffer> {
+public:
+    static NonnullRefPtr<KBuffer> create_with_size(size_t size)
+    {
+        auto region = MM.allocate_kernel_region(PAGE_ROUND_UP(size), "KBuffer");
+        ASSERT(region);
+        return adopt(*new KBuffer(*region, size));
+    }
+
+    static NonnullRefPtr<KBuffer> copy(const void* data, size_t size)
+    {
+        auto buffer = create_with_size(size);
+        memcpy(buffer->data(), data, size);
+        return buffer;
+    }
+
+    u8* data() { return m_region->vaddr().as_ptr(); }
+    const u8* data() const { return m_region->vaddr().as_ptr(); }
+    size_t size() const { return m_size; }
+    size_t capacity() const { return m_region->size(); }
+
+private:
+    explicit KBuffer(NonnullRefPtr<Region>&& region, size_t size)
+        : m_size(size)
+        , m_region(move(region))
+    {
+    }
+
+    size_t m_size { 0 };
+    NonnullRefPtr<Region> m_region;
+};