Net: Try to reuse incoming packet buffers to avoid allocation churn

The majority of the time in NetworkTask was being spent in allocating
and deallocating KBuffers for each incoming packet.

We'll now keep up to 100 buffers around and reuse them for new packets
if the next incoming packet fits in an old buffer. This is pretty
naively implemented but definitely cuts down on time spent here.

Kernel/Net/NetworkAdapter.cpp

@@ -1,11 +1,11 @@
 #include <AK/HashTable.h>
 #include <AK/StringBuilder.h>
+#include <Kernel/Heap/kmalloc.h>
 #include <Kernel/Lock.h>
 #include <Kernel/Net/EtherType.h>
 #include <Kernel/Net/EthernetFrameHeader.h>
 #include <Kernel/Net/NetworkAdapter.h>
 #include <Kernel/StdLib.h>
-#include <Kernel/Heap/kmalloc.h>
 
 static Lockable<HashTable<NetworkAdapter*>>& all_adapters()
 {
@@ -103,17 +103,42 @@ void NetworkAdapter::did_receive(const u8* data, int length)
     InterruptDisabler disabler;
     m_packets_in++;
     m_bytes_in += length;
-    m_packet_queue.append(KBuffer::copy(data, length));
+
+    Optional<KBuffer> buffer;
+
+    if (m_unused_packet_buffers.is_empty()) {
+        buffer = KBuffer::copy(data, length);
+    } else {
+        buffer = m_unused_packet_buffers.take_first();
+        --m_unused_packet_buffers_count;
+        if ((size_t)length <= buffer.value().size()) {
+            memcpy(buffer.value().data(), data, length);
+            buffer.value().set_size(length);
+        } else {
+            buffer = KBuffer::copy(data, length);
+        }
+    }
+
+    m_packet_queue.append(buffer.value());
+
     if (on_receive)
         on_receive();
 }
 
-Optional<KBuffer> NetworkAdapter::dequeue_packet()
+size_t NetworkAdapter::dequeue_packet(u8* buffer, size_t buffer_size)
 {
     InterruptDisabler disabler;
     if (m_packet_queue.is_empty())
-        return {};
-    return m_packet_queue.take_first();
+        return 0;
+    auto packet = m_packet_queue.take_first();
+    size_t packet_size = packet.size();
+    ASSERT(packet_size <= buffer_size);
+    memcpy(buffer, packet.data(), packet_size);
+    if (m_unused_packet_buffers_count < 100) {
+        m_unused_packet_buffers.append(packet);
+        ++m_unused_packet_buffers_count;
+    }
+    return packet_size;
 }
 
 void NetworkAdapter::set_ipv4_address(const IPv4Address& address)