blk-mq: optimise rq sort function

Check "!=" in multi-layer comparisons. The same memory usage, fewer
instructions, and 2 from 4 jumps are replaced with SETcc.

Note, that list_sort() doesn't differ 0 and <0.

Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>