SharedBuffer: Fix deadlock on destroy

We were locking the list of references, and then destroying the
reference, which made things go a little crazy.

It's more straightforward to just remove the per-reference lock: the
syscalls all have to lock the full list anyway, so let's just do that
and avoid the hassle.

While I'm at it, also move the SharedBuffer code out to its own file as it's
getting a little long and unwieldly, and Process.cpp is already huge.

Kernel/SharedBuffer.h

@@ -0,0 +1,50 @@
+#pragma once
+
+#include <Kernel/VM/MemoryManager.h>
+#include <AK/OwnPtr.h>
+
+struct SharedBuffer {
+private:
+    struct Reference {
+        Reference(pid_t pid)
+            : pid(pid)
+        {
+        }
+
+        pid_t pid;
+        unsigned count { 1 };
+        Region* region { nullptr };
+    };
+public:
+    SharedBuffer(int id, int size)
+        : m_shared_buffer_id(id)
+        , m_vmo(VMObject::create_anonymous(size))
+    {
+#ifdef SHARED_BUFFER_DEBUG
+        dbgprintf("Created shared buffer %d of size %d\n", m_shared_buffer_id, size);
+#endif
+    }
+
+    ~SharedBuffer()
+    {
+#ifdef SHARED_BUFFER_DEBUG
+        dbgprintf("Destroyed shared buffer %d of size %d\n", m_shared_buffer_id, size());
+#endif
+    }
+
+    bool is_shared_with(pid_t peer_pid);
+    void* get_address(Process& process);
+    void share_with(pid_t peer_pid);
+    void release(Process& process);
+    void disown(pid_t pid);
+    size_t size() const { return m_vmo->size(); }
+    void destroy_if_unused();
+    void seal();
+
+    int m_shared_buffer_id { -1 };
+    bool m_writable { true };
+    NonnullRefPtr<VMObject> m_vmo;
+    Vector<Reference, 2> m_refs;
+};
+
+Lockable<HashMap<int, OwnPtr<SharedBuffer>>>& shared_buffers();