[kernel] Fix logger::get_entry() blocking bug

The new logger event object for making get_entry() block when no logs
are available was consuming the event's notification even if the thread
did not need to block. This was causing excessive blocking - if multiple
logs had been added since the last call to get_entry(), only one would
be returned, and the next call would block until yet another log was
added.

Now only call event::wait() to block the calling thread if there are no
logs available.

src/kernel/logger.cpp

@@ -94,14 +94,18 @@ logger::output(level severity, logs area, const char *fmt, va_list args)
 size_t
 logger::get_entry(void *buffer, size_t size)
 {
-    m_event.wait();
 
     util::scoped_lock lock {m_lock};
 
     void *out;
     size_t out_size = m_buffer.get_block(&out);
+    if (out_size == 0 || out == 0) {
+        m_event.wait();
+        out_size = m_buffer.get_block(&out);
+
         if (out_size == 0 || out == 0)
             return 0;
+    }
 
     kassert(out_size >= sizeof(entry), "Couldn't read a full entry");
     if (out_size < sizeof(entry))