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/IPv4Socket.cpp

@@ -212,7 +212,8 @@ ErrorOr<size_t> IPv4Socket::sendto(OpenFileDescription&, UserOrKernelBuffer cons
         return set_so_error(EPIPE);
 
     auto allow_using_gateway = ((flags & MSG_DONTROUTE) || m_routing_disabled) ? AllowUsingGateway::No : AllowUsingGateway::Yes;
-    auto routing_decision = route_to(m_peer_address, m_local_address, bound_interface(), allow_using_gateway);
+    auto adapter = bound_interface().with([](auto& bound_device) -> RefPtr<NetworkAdapter> { return bound_device; });
+    auto routing_decision = route_to(m_peer_address, m_local_address, adapter, allow_using_gateway);
     if (routing_decision.is_zero())
         return set_so_error(EHOSTUNREACH);