blk-mq: allow resize of scheduler requests

Add support for growing the tags associated with a hardware queue, for
the scheduler tags. Currently we only support resizing within the
limits of the original depth, change that so we can grow it as well by
allocating and replacing the existing scheduler tag set.

This is similar to how we could increase the software queue depth with
the legacy IO stack and schedulers.

Signed-off-by: Jens Axboe <axboe@fb.com>
Reviewed-by: Omar Sandoval <osandov@fb.com>