io context: fix ref counting

Commit d9c7d394a8ebacb60097b192939ae9f15235225e
("block: prevent possible io_context->refcount overflow") mistakenly
changed atomic_inc(&ioc->nr_tasks) to atomic_long_inc(&ioc->refcount).

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Acked-by: Nikanth Karthikesan <knikanth@suse.de>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>