With new API blk_mq_is_reserved_rq() we can tell if a request is from
the reserved pool, so stop passing 'reserved' arg. There is actually
only a single user of that arg for all the callback implementations, which
can use blk_mq_is_reserved_rq() instead.
This will also allow us to stop passing the same 'reserved' around the
blk-mq iter functions next.