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/TCPSocket.h

@@ -189,7 +189,7 @@ private:
     HashMap<IPv4SocketTuple, NonnullRefPtr<TCPSocket>> m_pending_release_for_accept;
     Direction m_direction { Direction::Unspecified };
     Error m_error { Error::None };
-    LockRefPtr<NetworkAdapter> m_adapter;
+    SpinlockProtected<RefPtr<NetworkAdapter>, LockRank::None> m_adapter;
     u32 m_sequence_number { 0 };
     u32 m_ack_number { 0 };
     State m_state { State::Closed };
@@ -200,7 +200,7 @@ private:
 
     struct OutgoingPacket {
         u32 ack_number { 0 };
-        LockRefPtr<PacketWithTimestamp> buffer;
+        RefPtr<PacketWithTimestamp> buffer;
         size_t ipv4_payload_offset;
         LockWeakPtr<NetworkAdapter> adapter;
         int tx_counter { 0 };