[kernel] Rename stack_cache to buffer_cache

Renamed and genericized the stack_cache class to manage any address
range area of buffers or memory regions. Removed singleton and created
some globals that map to different address regions (kernel stacks,
kernel buffers).

Tags: vmem virtual memeory

src/kernel/buffer_cache.h

@@ -0,0 +1,29 @@
+#pragma once
+
+#include <kutil/vector.h>
+
+/// A cache of kernel stack address ranges
+class buffer_cache
+{
+public:
+	/// Constructor.
+	/// \args start  Start of virtual memory area to contain buffers
+	/// \args size   Size of individual buffers in bytes
+	/// \args length Size of virtual memory area in bytes
+	buffer_cache(uintptr_t start, size_t size, size_t length);
+
+	/// Get an available stack address
+	uintptr_t get_buffer();
+
+	/// Return a buffer address to the available pool
+	void return_buffer(uintptr_t addr);
+
+private:
+	kutil::vector<uintptr_t> m_cache;
+	uintptr_t m_next;
+	const uintptr_t m_end;
+	const size_t m_size;
+};
+
+extern buffer_cache g_kstack_cache;
+extern buffer_cache g_kbuffer_cache;