blk-wbt: Eliminate ambiguity in the comments of struct rq_wb

blk-wbt: Eliminate ambiguity in the comments of struct rq_wb

In the current implementation, the last_issue and last_comp members of
struct rq_wb are used only by read requests and not by non-throttled write
requests. Therefore, eliminate the ambiguity here.

Signed-off-by: Tang Yizhou <yizhou.tang@shopee.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20250727173959.160835-3-yizhou.tang@shopee.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>