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.cpp

@@ -111,9 +111,9 @@ size_t NetworkAdapter::dequeue_packet(u8* buffer, size_t buffer_size, Time& pack
     return packet_size;
 }
 
-LockRefPtr<PacketWithTimestamp> NetworkAdapter::acquire_packet_buffer(size_t size)
+RefPtr<PacketWithTimestamp> NetworkAdapter::acquire_packet_buffer(size_t size)
 {
-    auto packet = m_unused_packets.with([size](auto& unused_packets) -> LockRefPtr<PacketWithTimestamp> {
+    auto packet = m_unused_packets.with([size](auto& unused_packets) -> RefPtr<PacketWithTimestamp> {
         if (unused_packets.is_empty())
             return nullptr;
 
@@ -135,7 +135,7 @@ LockRefPtr<PacketWithTimestamp> NetworkAdapter::acquire_packet_buffer(size_t siz
     auto buffer_or_error = KBuffer::try_create_with_size("NetworkAdapter: Packet buffer"sv, size, Memory::Region::Access::ReadWrite, AllocationStrategy::AllocateNow);
     if (buffer_or_error.is_error())
         return {};
-    packet = adopt_lock_ref_if_nonnull(new (nothrow) PacketWithTimestamp { buffer_or_error.release_value(), kgettimeofday() });
+    packet = adopt_ref_if_nonnull(new (nothrow) PacketWithTimestamp { buffer_or_error.release_value(), kgettimeofday() });
     if (!packet)
         return {};
     packet->buffer->set_size(size);