gui: CpuConsumer::lockNextBuffer change return code when too many bufs acquired

- Return NOT_ENOUGH_DATA instead of INVALID_OPERATION when too many
  buffers have already been locked.
- INVALID_OPERATION is nominally used when something irrecoverable happens,
  but in this case the client just needs to call unlockBuffer to go back into a
  good state.

Bug: 10333400
Change-Id: I3a034d77de85741429f832a90eedd670afa1dc94
diff --git a/libs/gui/CpuConsumer.cpp b/libs/gui/CpuConsumer.cpp
index 3db46bc..bff55d1 100644
--- a/libs/gui/CpuConsumer.cpp
+++ b/libs/gui/CpuConsumer.cpp
@@ -72,7 +72,9 @@
 
     if (!nativeBuffer) return BAD_VALUE;
     if (mCurrentLockedBuffers == mMaxLockedBuffers) {
-        return INVALID_OPERATION;
+        CC_LOGW("Max buffers have been locked (%d), cannot lock anymore.",
+                mMaxLockedBuffers);
+        return NOT_ENOUGH_DATA;
     }
 
     BufferQueue::BufferItem b;