blk-mq: Invert check in blk_mq_update_nr_requests()

It's easier to read:

if (x)
	X;
else
	Y;

over:

if (!x)
	Y;
else
	X;

No functional change intended.

Signed-off-by: John Garry <john.garry@huawei.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Link: https://lore.kernel.org/r/1633429419-228500-5-git-send-email-john.garry@huawei.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>