nfp: flower: refine the use of circular buffer

nfp: flower: refine the use of circular buffer

The current use of circular buffer to manage stats_context_id is
very obscure, and it will cause problem if its element size is
not power of two. So change the use more straightforward and
scalable, and also change that for mask_id to keep consistency.

Signed-off-by: Yinjun Zhang <yinjun.zhang@corigine.com>
Signed-off-by: Simon Horman <simon.horman@corigine.com>
Link: https://lore.kernel.org/r/1639618621-5857-1-git-send-email-yinjun.zhang@corigine.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>