blk-mq-debugfs: rename hw queue directories from <n> to hctx<n>

It's not clear what these numbered directories represent unless you
consult the code. We're about to get rid of the intermediate "mq"
directory, so these would be even more confusing without that context.

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