block: make struct rq_list available for !CONFIG_BLOCK

A previous commit changed how requests are linked in the plug structure,
but unlike the previous method, it uses a new type for it rather than
struct request. The latter is available even for !CONFIG_BLOCK, while
struct rq_list is now. Move it outside CONFIG_BLOCK.

Reported-by: Nathan Chancellor <nathan@kernel.org>
Fixes: a3396b99 ("block: add a rq_list type")
Signed-off-by: Jens Axboe <axboe@kernel.dk>