[PATCH v2 1/2] bcache: writeback rate shouldn't artifically clamp
[PATCH v2 2/2] bcache: rearrange writeback main thread ratelimit

This is a reroll of the previous "don't clamp" patch.  It corrects
type issues where negative numbers were handled badly (mostly for
display in writeback_rate_debug).

Additionally, a new, related patch-- during scanning for dirty
blocks, don't reset the ratelimiting counter.  This can prevent
undershoots/overshoots of the target rate relating to scanning.

Mike