Change pingpong index and names to distinguish between general use
pingpong blocks and pingpong blocks dedicated for concurrent writeback

Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Jessica Zhang <quic_jesszhan@quicinc.com>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/629229/
Link: https://lore.kernel.org/r/20241216-concurrent-wb-v4-10-fe220297a7f0@quicinc.com