staging: ion: WARN when the handle kmap_cnt is going to wrap around

There are certain client bugs (double unmap, for example) that can cause
the handle->kmap_cnt (an unsigned int) to wrap around from zero. This
causes problems when the handle is destroyed because we have:

        while (handle->kmap_cnt)
                ion_handle_kmap_put(handle);

which takes a long time to complete when kmap_cnt starts at ~0 and can
result in a watchdog timeout.

WARN and bail when kmap_cnt is about to wrap around from zero.

Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>
Acked-by: Colin Cross <ccross@android.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/android/ion/ion.c

@@ -627,6 +627,10 @@ static void ion_handle_kmap_put(struct ion_handle *handle)
 {
 	struct ion_buffer *buffer = handle->buffer;
 
+	if (!handle->kmap_cnt) {
+		WARN(1, "%s: Double unmap detected! bailing...\n", __func__);
+		return;
+	}
 	handle->kmap_cnt--;
 	if (!handle->kmap_cnt)
 		ion_buffer_kmap_put(buffer);