alloc_workqueue replaces deprecated create_workqueue().

A dedicated workqueue has been used since the workitem (viz
&fault->work), is involved in IO page-fault handling.
WQ_MEM_RECLAIM has been set to guarantee forward progress under memory
pressure, which is a requirement here.
Since there are only a fixed number of work items, explicit concurrency
limit is unnecessary.
Signed-off-by: Bhaktipriya Shridhar <bhaktipriya96@gmail.com>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Joerg Roedel <jroedel@suse.de>