Kernel: Lock socket mutex across {get,set}sockopt() and SO_ERROR updates

Since a socket can be accessed by multiple threads concurrently, we need
to protect shared data behind the socket mutex.

There's very likely more places where we need to fix this, the purpose
of this patch is to fix a VERIFY() failure in getsockopt() seen on CI.

Kernel/Net/Socket.h

@@ -130,16 +130,22 @@ protected:
 
     Role m_role { Role::None };
 
-    ErrorOr<void> so_error() const { return m_so_error; }
+    ErrorOr<void> so_error() const
+    {
+        VERIFY(m_mutex.is_locked_by_current_thread());
+        return m_so_error;
+    }
 
     Error set_so_error(ErrnoCode error_code)
     {
+        MutexLocker locker(mutex());
         auto error = Error::from_errno(error_code);
         m_so_error = error;
         return error;
     }
     Error set_so_error(Error error)
     {
+        MutexLocker locker(mutex());
         m_so_error = error;
         return error;
     }