Kernel: Add DoubleBuffer::try_create() factory method for OOM hardening

We need to expose the ability for DoubleBuffer creation to expose
failure, as DoubleBuffer depends on KBuffer, which also has to be able
to expose failure during OOM.

We will remove the non OOM API once all users have been converted.

Kernel/DoubleBuffer.h

@@ -16,8 +16,8 @@ namespace Kernel {
 
 class DoubleBuffer {
 public:
+    [[nodiscard]] static OwnPtr<DoubleBuffer> try_create(size_t capacity = 65536);
     explicit DoubleBuffer(size_t capacity = 65536);
-
     [[nodiscard]] KResultOr<size_t> write(const UserOrKernelBuffer&, size_t);
     [[nodiscard]] KResultOr<size_t> write(const u8* data, size_t size)
     {
@@ -47,6 +47,7 @@ public:
     }
 
 private:
+    explicit DoubleBuffer(size_t capacity, NonnullOwnPtr<KBuffer> storage);
     void flip();
     void compute_lockfree_metadata();
 
@@ -60,7 +61,7 @@ private:
     InnerBuffer m_buffer1;
     InnerBuffer m_buffer2;
 
-    KBuffer m_storage;
+    NonnullOwnPtr<KBuffer> m_storage;
     Function<void()> m_unblock_callback;
     size_t m_capacity { 0 };
     size_t m_read_buffer_index { 0 };