Kernel/Net: Iron out the locking mechanism across the subsystem

There is a big mix of LockRefPtrs all over the Networking subsystem, as
well as lots of room for improvements with our locking patterns, which
this commit will not pursue, but will give a good start for such work.

To deal with this situation, we change the following things:
- Creating instances of NetworkAdapter should always yield a non-locking
  NonnullRefPtr. Acquiring an instance from the NetworkingManagement
  should give a simple RefPtr,as giving LockRefPtr does not really
  protect from concurrency problems in such case.
- Since NetworkingManagement works with normal RefPtrs we should
  protect all instances of RefPtr<NetworkAdapter> with SpinlockProtected
  to ensure references are gone unexpectedly.
- Protect the so_error class member with a proper spinlock. This happens
  to be important because the clear_so_error() method lacked any proper
  locking measures. It also helps preventing a possible TOCTOU when we
  might do a more fine-grained locking in the Socket code, so this could
  be definitely a start for this.
- Change unnecessary LockRefPtr<PacketWithTimestamp> in the structure
  of OutgoingPacket to a simple RefPtr<PacketWithTimestamp> as the whole
  list should be MutexProtected.

Kernel/Net/NetworkAdapter.h

@@ -39,7 +39,7 @@ struct PacketWithTimestamp final : public AtomicRefCounted<PacketWithTimestamp>
 
     NonnullOwnPtr<KBuffer> buffer;
     Time timestamp;
-    IntrusiveListNode<PacketWithTimestamp, LockRefPtr<PacketWithTimestamp>> packet_node;
+    IntrusiveListNode<PacketWithTimestamp, RefPtr<PacketWithTimestamp>> packet_node;
 };
 
 class NetworkingManagement;
@@ -91,7 +91,7 @@ public:
     u32 packets_out() const { return m_packets_out; }
     u32 bytes_out() const { return m_bytes_out; }
 
-    LockRefPtr<PacketWithTimestamp> acquire_packet_buffer(size_t);
+    RefPtr<PacketWithTimestamp> acquire_packet_buffer(size_t);
     void release_packet_buffer(PacketWithTimestamp&);
 
     constexpr size_t layer3_payload_offset() const { return sizeof(EthernetFrameHeader); }