GFS2: Use new workqueue scheme

The recovery workqueue can be freezable since
we want it to finish what it is doing if the system is to
be frozen (although why you'd want to freeze a cluster node
is beyond me since it will result in it being ejected from
the cluster). It does still make sense for single node
GFS2 filesystems though.

The glock workqueue will benefit from being able to run more
work items concurrently. A test running postmark shows
improved performance and multi-threaded workloads are likely
to benefit even more. It needs to be high priority because
the latency directly affects the latency of filesystem glock
operations.

The delete workqueue is similar to the recovery workqueue in
that it must not get blocked by memory allocations, and may
run for a long time.

Potentially other GFS2 threads might also be converted to
workqueues, but I'll leave that for a later patch.

Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>
Acked-by: Tejun Heo <tj@kernel.org>